[clang] [clang] Split up `SemaDeclAttr.cpp` (PR #93966)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I didn't take a close look at the individual .cpp files, I assume that is mostly just Copy  & paste.

I would like us to be a little more forward-decl aggressive in the headers however, and perhpas try to minimize what gets pushed up to Sema.h.

https://github.com/llvm/llvm-project/pull/93966