[clang] [Parse] Remove ParseDiagnostic.h (PR #116496)

[Kazu Hirata via cfe-commits]

kazutakahirata wrote:

> Can you explain what is the perceived benefit of this change? Thanks

Sure.

- We'll have less code.
- We'll have fewer "false positives" from clangd's "unused include" warnings.  The .cpp files are not using anything declared in `ParseDiagnostic.h` because, after all, `ParseDiagnostic.h` does not declare anything in C++ (classes, functions, templates, etc).

Is there a (possibly unwritten) policy of some sort for `Parse/*.cpp` to use `ParseDiagnostic.h` instead of `DiagnosticParse.h` so that, for example, we can add things to `ParseDiagnostic.h` and get them shared instantly among `Parse/*.cpp`?


https://github.com/llvm/llvm-project/pull/116496