[libc++] Allow running the test suite with optimizations (#68753)

This patch adds a configuration of the libc++ test suite that enables
optimizations when building the tests. It also adds a new CI
configuration to exercise this on a regular basis. This is added in the
context of [1], which requires building with optimizations in order to
hit the bug.

[1]: https://github.com/llvm/llvm-project/issues/68552