Implement feature test macros using a script.

Implement feature test macros using a script.

Summary:
This patch implements all the feature test macros libc++ currently supports, as specified by the standard or cppreference prior to C++2a.

The tests and `<version>` header are generated using a script. The script contains a table of each feature test macro, the headers it should be accessible from, and its values of each dialect of C++.
When a new feature test macro is added or needed, the table should be updated and the script re-run.

Reviewers: mclow.lists, jfb, serge-sans-paille

Reviewed By: mclow.lists

Subscribers: arphaman, jfb, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D56750

llvm-svn: 351286