[clang] [clang-tools-extra] [Clang] Treat line directives as header names. (PR #211512)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman this PR is _not_ accepting . As i said, I don't think we should, no one else is

Oh, duh, I was thrown by the change to using `LexHeaderName()` but I see we're then checking the form of the header name we lexed. Nevermind the angle bracket suggestions, sorry for the noise!

Should we test something like:
```
#define FILENAME "foo.c"

#line 12 FILENAME
```
(if we're going back to the source text to look for what it says, are we seeing it before or after macro expansions?)
https://godbolt.org/z/Pfqeboxro

https://github.com/llvm/llvm-project/pull/211512