[clang] [Clang][CodeGen] Lookup standard library declarations for builtins to… (PR #204792)

[Hans Wennborg via cfe-commits]

zmodem wrote:

I'm not sure I agree. I think the intention of these builtins is to call the CRT directly, without requiring the header, but this code was added well before the MSVC support, so it didn't take that into account.

Unless there is something in the CRT which makes it not suitable for calling directly, I think we should go ahead and always call e.g. `_hypotf` with dllimport when targeting a windows-msvc triple, regardless of whether the header was included or not.

https://github.com/llvm/llvm-project/pull/204792