Summary:
This patch adds a very basic `FILE` type and basic `fwrite`.
It also removes `snprintf` from `StdIO`'s function spec because `VarArgType` was causing the generation to fail.
Reviewers: sivachandra, PaulkaToast
Reviewed By: sivachandra
Subscribers: mgorny, MaskRay, tschuett, libc-commits
Differential Revision: https://reviews.llvm.org/D77626