[libc] Add very basic stdio FILE and fwrite
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