[libc++] Transition several remaining generated tests to the new .gen format

In addition to reducing the amount of boilerplate we need to generate
whenever a new header is added, this also improves the existing tests
by running them in separate Lit tests (so they can be parallelized).
This also creates separate translation units for most header tests,
which is what we really should have done from the start since it
isolates each header we're testing.

Differential Revision: https://reviews.llvm.org/D151654