[libc] add result class to strtointeger

[libc] add result class to strtointeger

This is a class intended to improve errno handling for internal
functions by allowing functions to return their result and error status
instead of setting errno. This specific class will be used for
strtointeger and (in a followup patch) strtofloat.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D133163