[clangd] cleanup of header guard names

[clangd] cleanup of header guard names

Renaming header guards to match the LLVM convention.
This patch was created by automatically applying the fixes from
clang-tidy.

I've removed the [NFC]  tag from the title, as we're adding header guards in some files and thus might trigger behavior changes.

Differential Revision: https://reviews.llvm.org/D113896