[clang] [clang] Do not clear FP pragma stack when instantiating functions (PR #70646)

[John McCall via cfe-commits]

rjmccall wrote:

I see, that makes sense.  It shouldn't really need to be saved even during late template parsing, right?  Late template parsing is never at the top level, and push/pop are only allowed at the top level.

https://github.com/llvm/llvm-project/pull/70646