This introduces gmtime to LLVM libc, based on C99/C2X/Single Unix Spec.

This change doesn't handle TIMEZONE, tm_isdst and leap seconds.

Moved shared code between mktime and gmtime into time_utils.cpp.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D98467

GitOrigin-RevId: eaae52c1fd459f9c0a147361bc3b962238faba5c
12 files changed