Hard-float (unlike mips32 musl targets but consistent with any other
musl target), MIPS64r2, n64 ABI.

The triples are renamed to carry the `abi64` ABI suffix found on all
other MIPS64 targets, for consistency and forward compatibility, should
Rust gain support for the n32 ABI one day.

This is a first attempt of adding support for the new Apple Catalyst ABI (i.e. running iOS apps on macOS). Currently, `rustc` supports the iOS and iOS simulator targets for iOS:
- iOS: ARM cpu, iOS SDK, linked agains the iOS ABI
- Simulator: X86_64 cpu, iOS SDK, linked against the iOS ABI

Apple Catalyst will add an additional target:
- Macabi: X86_64 CPU, iOS SDK, linked again the macOS ABI.

Note, it the actual SDK is the also the macOS 10.15 SDK, but the symbols are the iOS SDK symbols as they were added to macOS with 10.15.

This commits adds the files for this new target triple.

…tring`

Add UWP MSVC targets

Hi,

- The README URI change is the correct one for VS2019 community edition, which I suspect most people would use. Doesn't _need_ to be merged though.
- This rust-lang@5e6619e fixes the UWP build (msvc or not, doesn't matter). I suspect it broke with recent changes unnoticed because no CI.
- Store lib location is found through the VCToolsInstallDir env variable. The end of the path is currently for the VS2019 store lib locations only.
- I could not test the aarch64_uwp_windows_msvc target because the rust build script does not currently support arm64 msvc AFAIU.

…ichton

Add builtin targets for mips64(el)-unknown-linux-muslabi64

This is prerequisite for rust-lang/libc#1449.

Tested locally to produce working static and dynamic binaries, ~~but CI config is untested for now~~ CI is to be added in a follow-up PR.

*edit: dynamic binaries also confirmed working!*

*edit 2: changed triples to include ABI, and removed stray `crt_static_default = false` declarations to be consistent with other musl targets*

Adapt AddRetag for shallow retagging

With rust-lang/miri#872, Miri only retags "bare" references, not those nested in compound types. This adjust `Retag` statement generation to don't emit retags if they are definitely not a bare reference.

I also expanded the mir-opt test to cover the `Retag` in the drop shim, which had previously not been tested.

…bi-target-triple, r=estebank

Add Catalyst (iOS apps running on macOS) target

This is a first attempt of adding support for the new [Apple Catalyst](https://developer.apple.com/ipad-apps-for-mac/) target (i.e. running iOS apps on macOS). Currently, `rustc` supports the iOS and iOS simulator targets for iOS:
- iOS: ARM cpu, iOS SDK, linked agains the iOS ABI
- Simulator: X86_64 cpu, iOS SDK, linked against the iOS ABI

Apple Catalyst will add an additional target:
- Macabi: X86_64 CPU, iOS SDK, linked again the macOS ABI.

Note, it the actual SDK is the also the macOS 10.15 SDK, but the symbols are the iOS SDK symbols as they were added to macOS with 10.15.

I've collected additional information via links in the open question sections below. This is way out of my comfort zone so please excuse whatever errors I may have made.

# Open Questions:

## Clang Version
It seems to me that `macabi` has not been merged into `clang` yet, I don't know whether that is a requirement rustc to compile, or if it is sufficient if the Clang that is used on a developers system is the correct one supporting macabi (that comes with current Xcode)

## Hardcoded iOS version

`swift-llvm` actually used [x86_64-apple-ios13.0-macabi](apple/swift-llvm@3f1fd4f) as the target triple which hard-codes the current iOS version. A post on stackoverflow [points out that `MIN_IOS_VERSION` and `MIN_OSX_VERSION` should be used when compiling C code for clang (`-target x86_64-apple-ios${MIN_IOS_VERSION}-macabi`)](https://stackoverflow.com/questions/56487645/how-to-compile-a-3rd-party-library-to-be-used-with-uikit-for-mac-catalyst). However, I wasn't entirely sure how to do that in this PR. Pointers welcome.

## Data Layout
I'm probably using the wrong data-layout. I don't know whether it should be the macOS version or the iOS version. This is probably easier to answer for somebody who understands these things much better than me. I just copied the iOS Simulator X86_64 version as it seems to be (based on what I understand) that Catalyst is just the simulator target build against a different SDK.

# Current State
1. I got it to compile
2. I could successfully compile a `macabi` `libcore` via `cargo build --target x86_64-apple-ios-macabi`

I'm not sure what needs to be done next. Supposedly I need to compile everything into a toolchain somehow that I can then test via `rustup` to make sure that a binary compiled against the toolchain also works with Catalyst. [I read this article, but I'm still lost](https://www.reddit.com/r/rust/comments/5ag60z/how_do_i_bootstrap_rust_to_crosscompile_for_a_new/d9gicr2/) and would love pointers what to do next here.

# Additional Information
- [Commit adding Catalyst support to the Swift Clang Fork](CocoaPods/CocoaPods#8877)
- [Compiling C to Catalyst Discussion](CocoaPods/CocoaPods#8877)
- [CocoaPods Discussion on Adding Catalyst support](CocoaPods/CocoaPods#8877)

…fJung

Remove uses of `mem::uninitialized()` from cloudabi

This PR removes uses of `mem::uninitialized` from `cloudabi` module,
excluding the layout test in `src/libstd/sys/cloudabi/abi/cloudabi.rs`.

r? @RalfJung
cc @EdSchouten
cc rust-lang#62397

remove unused Level::PhaseFatal

r? @estebank

Test HRTB issue accepted by compiler

Hi! First Rust PR, so if anything needs changing just let me know and I'll take care of it right away.

Closes rust-lang#50301 which was marked E-needstest

Fix ICE rust-lang#63226

Fixes rust-lang#63226

r? @oli-obk

cleanup: Remove `Spanned` where possible

It generally only makes sense on enums, otherwise it's more convenient to "flatten" it by adding a span field to the struct it wraps.