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

[John McCall via cfe-commits]

https://github.com/rjmccall commented:

To be clear, we're still ensuring the current pragma state during instantiation is what it was when the template is parsed, we're just not incorrectly dropping all of the saved pragma state when we're doing it?

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