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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> And contrariwise, if there's some sneaky way to put push/pop pragmas in non-file contexts, that also seems like a serious problem, because the way we process them is not designed to understand local scopes, which means we're just doing random stuff instead of implementing any sort of intelligible language design.

What if it's a non-sneaky way? ;-) C has several fp-related pragmas that are required to be supported at block scope. For example, the specification for `#pragma STDC FP_CONTRACT` (C23 7.12.2p2) says: 

> ... Each pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. ...

Other standard pragmas have similar wording. CC @zahiraam who has also been working on pragmas in this area.

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