reland of https://github.com/llvm/llvm-project/pull/98215
Additionally adds proxy headers for FILE and the fopencookie types
The arm32 build has been failing due to redefinitions of the off_t type.
This patch fixes this by moving off_t to a proper proxy header. To do
this, it also moves stdio macros to a proxy header to hopefully avoid
including this proxy header alongside this public stdio.h.
In a previous patch, the printf writer was rewritten to use a single
writer class with a buffer and a callback hook. This patch refactors
scanf's reader to match conceptually.
This patch adds scanf, sscanf, and fscanf entrypoints. It also adds unit
tests for sscanf and a basic test to fscanf. The scanf function is
basically impossible to test in an automated fashion due to it recieving
user input.
Reviewed By: sivachandra, lntue
Differential Revision: https://reviews.llvm.org/D138076
