…nonymous lifetime parameter

Adding diagnostic code 0611 for lifetime errors with one named, one anonymous lifetime parameter

This is a fix for rust-lang#42517
Note that this only handles the above case for **function declarations** and **traits**.
`impl items` and `closures` will be handled in a later PR.
Example
```
fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32 {
    if x > y { x } else { y }
}
```
now displays the following error message. ui tests have been added for the same.
```
error[E0611]: explicit lifetime required in the type of `x`
11 | fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32 {
   |                     ^ consider changing the type of `x` to `&'a i32`
12 |     if x > y { x } else { y }
   |                  - lifetime `'a` required
```
rust-lang#42516
r? @nikomatsakis

Add pager support for `rustc --explain EXXXX`

Hello!

This is my take on rust-lang#32665.

Thanks!

**EDIT:** _I've limited access to a Windows machine so this is taking longer than I've anticipated_. 🐢

cc @alexcrichton @nikomatsakis @Mark-Simulacrum @retep998 @ollie27 @afiune

…r=GuillaumeGomez

Document possible `io::ErrorKind`s of `fs::open`

Try to make clear that this isn't an API guarantee for now, as we likely
want to refine these errors in the future, e.g. `ENOSPC` "No space left
on device".

CC rust-lang#40322

…sakis

Add E0619 error explanation

r? @eddyb

…ulacrum

rustc_llvm: re-run build script when env var LLVM_CONFIG changes

This removes the changes done in rust-lang#42429 and use the newly introduced `cargo:rerun-if-env-changed` in rust-lang/cargo#4125.
As `LLVM_CONFIG` env var points to the `llvm-config` and changes when it gets configured in `config.toml` or removed from it, we can re-run the build script if this env var changes.

closes rust-lang#42444

r? @alexcrichton

…imulacrum

Track `iterator_for_each` in rust-lang#42986

None