build: always revert `#pragma GCC diagnostic` after use

build: always revert `#pragma GCC diagnostic` after use

Before this patch some source files were overriding gcc warning options,
but without restoring them at the end of the file. In CMake UNITY builds
these options spilled over to the remainder of the source code,
effecitvely disabling them for a larger portion of the codebase than
intended.

`#pragma clang diagnostic` didn't have such issue in the codebase.

Reviewed-by: Marcel Raad
Closes #12352