[libc] Add ability to generate enum types/values to HdrGen.

A target to generate the std C threads.h file has been added. This
utilizes the new feature added in this change.

Reviewers: phosek

Differential Revision: https://reviews.llvm.org/D75379

GitOrigin-RevId: d1536673c68d5010d2a6a3a12a9dd205e8fca2d4
9 files changed