[libc] Add Month enum, refactor mktime_test

Introduce Month enum.

Use designated initializers instead of comments to ensure that tm_data
is initialized correctly.  Use Month enum for initializing .tm_mon to
avoid off-by-one errors.

Co-authored-by: Raman Tenneti <rtenneti@google.com>

Reviewed By: rtenneti

Differential Revision: https://reviews.llvm.org/D144357