This adds the unstable `-Z external-sanitizer-runtime` flag that will
prevent rustc from emitting linker paths for the in-tree LLVM sanitizer
runtime library.

…michaelwoerister

Add `-Z external-clangrt`

This adds the unstable `-Z external-clangrt` flag that will prevent rustc from emitting linker paths for the in-tree LLVM sanitizer runtime library.

diagnostics: suggest `Clone` bounds when noop `clone()`

Fixes rust-lang#121524

preserve span when evaluating mir::ConstOperand

This lets us show to the user where they were using the faulty const (which can be quite relevant when generics are involved).

I wonder if we should change "erroneous constant encountered" to something like "the above error was encountered while evaluating this constant" or so, to make this more similar to what the collector emits when showing a "backtrace" of where things get monomorphized? It seems a bit strange to rely on the order of emitted diagnostics for that but it seems the collector already [does that](https://github.com/rust-lang/rust/blob/da8a8c9223722e17cc0173ce9490076b4a6d263d/compiler/rustc_monomorphize/src/collector.rs#L472-L475).

…ompiler-errors

Pass the correct DefId when suggesting writing the aliased Self type out

Fixes rust-lang#122467.

…types, r=oli-obk

Ensure RPITITs are created before def-id freezing

From the test:

```rust
// `ty::Error` in a trait ref will silence any missing item errors, but will also
// prevent the `associated_items` query from being called before def ids are frozen.
```

Essentially, the code that checks that `impl`s have all their items (`check_impl_items_against_trait`) is also (implicitly) responsible for fetching the `associated_items` query before, but since we early return here:
https://github.com/rust-lang/rust/blob/c2901f543577af99b9cb708f5c0d28525eb7f08f/compiler/rustc_hir_analysis/src/check/check.rs#L732-L737
...that means that this never happens for trait refs that reference errors.

Fixes rust-lang#122518
r? oli-obk

Docs for `thir::ExprKind::Use` and `thir::ExprKind::Let`

These docs are based on my own recent investigations; hopefully they're reasonably accurate.

`Use` was particularly puzzling to me at first, since the name is not very suggestive, and the old docs were quite cryptic.

…=compiler-errors

Clean up AstConv

Split off from rust-lang#120926 to make it only contain the renaming & (doc) comment updates.
Any changes other than that which have accumulated over time are now part of this PR.
Let's be disciplined ;) Inspired by rust-lang#120926 (comment).

---

* Remove `hir_trait_to_predicates`
  * Unused since rust-lang#113671
* Inline `create_args_for_ast_trait_ref`
  * Only had a single call site
  * Having it as a separate method didn't gain us anything
* Use an if-let guard somewhere to avoid unwrapping
* Avoid explicit trait object lifetimes
  * More legible, stylistic-only (the updated code is 100% semantically identical)
  * Use explicitly elided lifetimes in impl headers, they get elaborated to distinct lifetimes
  * Make use of [object lifetime defaulting](https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes) for a trait object type inside of a reference type somewhere
* Use preexisting dedicated method `ItemCtxt::to_ty` over `<dyn AstConv<'_>>::ast_ty_to_ty`
* Use preexisting dedicated method `AstConv::astconv` over explicit coercions
* Simplify the function signature of `create_args_for_ast_path` and of `check_generic_arg_count`
  * In both cases redundant information was passed rendering the call sites verbose and confusing
  * No perf impact (tested in [rust-lang#120926](rust-lang#120926))
* Move diagnostic method `report_ambiguous_associated_type` from `astconv` to `astconv::errors`
  * The submodule `errors` exists specifically for that purpose
  * Use it to keep the main module clean & short