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

[Serge Pavlov via cfe-commits]

spavloff wrote:

Late parsing (in contrast to template instantiation) is sensitive to the pragma stack because the late parsed text may contain pragmas. If, for example, the parsed text contains unbalanced pus/pop pragmas, it is detected if the pragma stack is empty, but can be missed if there is an unfinished pragma.

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