[libc++] Use Lit annotations for all .gen.py tests

[libc++] Use Lit annotations for all .gen.py tests

Instead of guarding header tests using #ifdefs inside the tests,
use Lit markup to mark all the tests as unsupported. This is simpler
but also provides better feedback about which tests are being run
when running the test suite.

Differential Revision: https://reviews.llvm.org/D151893