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

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Would it make sense to add a new header (SemaUtils.h? SemaTargetUtils.h?) to move most of the exposed templated helpers to instead of Sema.h?

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