Using `#![feature(trivial_bounds)]`, it's possible to write functions
with unsatisfiable 'where' clauses, making them uncallable. However, the
user can act as if these 'where' clauses are true inside the body of the
function, leading to code that would normally be impossible to write.

Since const propgation can run even without any user-written calls to a
function, we need to explcitly check for these uncallable functions.

This avoids a strange linker error that we get with only "--emit=mir"
and "check-pass"

…s̶dataflow, r=ecstatic-morse

Document more use cases of dataflow

r? @ecstatic-morse

…, r=matthewjasper

Don't run const propagation on items with inconsistent bounds

Fixes rust-lang#67696

Using `#![feature(trivial_bounds)]`, it's possible to write functions
with unsatisfiable 'where' clauses, making them uncallable. However, the
user can act as if these 'where' clauses are true inside the body of the
function, leading to code that would normally be impossible to write.

Since const propgation can run even without any user-written calls to a
function, we need to explcitly check for these uncallable functions.

rustdoc: improve stability mark arrows

### current

![old-stability-arrow](https://user-images.githubusercontent.com/346530/71863520-134d8b00-3138-11ea-86f9-a98068b3cff9.png)

### new

![new-stability-arrow](https://user-images.githubusercontent.com/346530/71863539-1b0d2f80-3138-11ea-843e-d79b9e5d9eec.png)

### new dark

![dark-stability-arrow](https://user-images.githubusercontent.com/346530/71863563-26f8f180-3138-11ea-9514-050e2c779f90.png)

libterm: parse extended terminfo format

Fixes rust-lang#45728.

Modifies libterm to parse the extended terminfo format introduced in ncurses 6.1. This fixes the lack of color in test output for users with newer ncurses versions.

The ideal fix for this would be to migrate libtest to use `termcolor` (rust-lang#60349), but that's blocked for the foreseeable future.

Clean up some diagnostics by making them more consistent

In general:

- Diagnostic should start with a lowercase letter.
- Diagnostics should not end with a full stop.
- Ellipses contain three dots.
- Backticks should encode Rust code.

I also reworded a couple of messages to make them read more clearly.

It might be sensible to create a style guide for diagnostics, so these informal conventions are written down somewhere, after which we could audit the existing diagnostics.

r? @Centril

…ities, r=Centril

restore some rustc_parse visibilities for rustfmt

In rust-lang@c189565 some visibilities were reduced on the parse mod (which now resides in the rustc_parse crate) as part of some refactoring and splitting up of libsyntax. However, rustfmt needs access to a few of those items that are no longer visible.

This restores the visibility on those items rustfmt depends on.

rust-lang/rustfmt#3903 (comment)
rust-lang/rustfmt#4009

cc @topecongiro