[MSVC] Add support for pragma function

MSVC pragma function tells the compiler to generate calls to functions in the pragma function list, instead of using the builtin. Needs https://reviews.llvm.org/D124701

https://docs.microsoft.com/en-us/cpp/preprocessor/function-c-cpp?view=msvc-170

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D124702