[clang] [clang] Fix `gnu::init_priority` attribute handling for reserved values (PR #121577)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

We talked about this during my morning office hours today (thanks @erichkeane and @philnik777 for coming!) and I'm now comfortable with the approach taken in this patch. Warning which defaults to error strikes a pretty reasonable compromise.

https://github.com/llvm/llvm-project/pull/121577