[libc++] Migrate warning flags to the DSL

[libc++] Migrate warning flags to the DSL

This makes us closer to running the test suite on platforms where the
legacy test suite configuration doesn't work.

One notable change after this commit is that the tests will be run with
warnings enabled on GCC too, which wasn't the case before. However,
previous commits should have tweaked the test suite to make sure it
passes with warnings enabled on GCC.

Note that warnings can still be disabled with `--param enable_warnings=False`,
as before.

Differential Revision: https://reviews.llvm.org/D90432