[clangd] Cleanup of readability-identifier-naming

Auto-generated patch based on clang-tidy readability-identifier-naming.
Only some manual cleanup for `extern "C"` declarations and a GTest change was required.

I'm not sure if this cleanup is actually very useful. It cleans up clang-tidy findings to the number of warnings from clang-tidy should be lower.  Since it was easy to do and required only little cleanup I thought I'd upload it for discussion.

One pattern that keeps recurring: Test **matchers** are also supposed to start with a lowercase letter as per LLVM convention. However GTest naming convention for matchers start with upper case. I would propose to keep stay consistent with the GTest convention there. However that would imply a lot of `//NOLINT` throughout these files.

To re-product this patch run:
```
run-clang-tidy -checks="-*,readability-identifier-naming" -fix -format ./clang-tools-extra/clangd
```

To convert the macro names, I was using this script with some manual cleanup afterwards:
https://gist.github.com/ChristianKuehnel/a01cc4362b07c58281554ab46235a077

Differential Revision: https://reviews.llvm.org/D115634