Suggest adding a new lifetime parameter when two elided lifetimes should match up but don't

Issue rust-lang#90170

This also changes the tests introduced by the previous commits because of another rustc issue (rust-lang#90258)

And add a rustdoc-gui test confirming various header sizes.

`is_clobber()` already checks if `reg` is a register and the both
values should be the same.

Properly check `target_features` not to trigger an assertion

Fixes rust-lang#89875
I think it should be a condition instead of an assertion to check if it's a register as it's possible that `reg` is a register class.
Also, this isn't related to the issue directly, but `is_target_supported` doesn't check `target_features` attributes. Is there any way to check it on rustc_codegen_llvm?

r? `@Amanieu`

…-hint, r=estebank

Add beginner friendly lifetime elision hint to E0623

Address rust-lang#90170

Suggest adding a new lifetime parameter when two elided lifetimes should match up but don't.

Example:

```
error[E0623]: lifetime mismatch
  --> $DIR/issue-90170-elision-mismatch.rs:2:35
   |
LL | fn foo(slice_a: &mut [u8], slice_b: &mut [u8]) {
   |                 ---------           --------- these two types are declared with different lifetimes...
LL |     core::mem::swap(&mut slice_a, &mut slice_b);
   |                                   ^^^^^^^^^^^^ ...but data from `slice_b` flows into `slice_a` here
   |
   = note: Each elided lifetime in input position becomes a distinct lifetime.
help: Explicitly declare a lifetime and assign it to both
   |
LL | fn foo<'a>(slice_a: &'a mut [u8], slice_b: &'a mut [u8]) {
   |       ++++           ++                     ++

```

for

```rust
fn foo(slice_a: &mut [u8], slice_b: &mut [u8]) {
    core::mem::swap(&mut slice_a, &mut slice_b);
}
```

…omez

Fix documentation header sizes

And add a rustdoc-gui test confirming various header sizes.

Split off from rust-lang#90156. This fixes a regression in rust-lang#89506 where the heading level of titles within Markdown was too high (h2) for docblocks under structs, unions, and enum impls.

r? `@camelid`

Demo: https://jacob.hoffman-andrews.com/rust/fix-header-sizes/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E
Stable: https://doc.rust-lang.org/stable/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E
Beta: https://doc.rust-lang.org/beta/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E

rustdoc: Use TTF based font instead of OTF for CJK glyphs to improve readability

Due to Windows' implementation of font rendering, OpenType fonts can be distorted. So the existing font, Noto Sans KR, is not very readable on Windows. This PR improves readability of Korean glyphs on Windows.

## Before
![원1](https://user-images.githubusercontent.com/11029378/138592394-16b15787-532d-4421-a5eb-ed85675290fa.png)

## After
![원2](https://user-images.githubusercontent.com/11029378/138592409-f3a440ee-f0fc-40e4-9561-42c479439c9f.png)

The fonts included in this PR are licensed under the SIL Open Font License and generated with these commands:

```sh
pyftsubset NanumBarunGothic.ttf \
--unicodes=U+AC00-D7AF,U+1100-11FF,U+3130-318F,U+A960-A97F,U+D7B0-D7FF \
--output-file=NanumBarunGothic.ttf.woff --flavor=woff
```
```sh
pyftsubset NanumBarunGothic.ttf \
--unicodes=U+AC00-D7AF,U+1100-11FF,U+3130-318F,U+A960-A97F,U+D7B0-D7FF \
--output-file=NanumBarunGothic.ttf.woff2 --flavor=woff2
```

r? `@GuillaumeGomez`