[libc++] Add support for a new keyword ADDITIONAL_COMPILE_FLAGS

[libc++] Add support for a new keyword ADDITIONAL_COMPILE_FLAGS

This allows adding compilation flags for a single test, which can help
eliminate some .sh.cpp tests and some custom handling in the libc++
test format.

It also works around the issue that .sh.cpp substitutions are _not_
equivalent to the actual compiler command lines used to compile tests,
since the compiler flags can be modified in local lit configurations,
and substitutions are frozen at that point. For example using %{compile}
in a .sh.cpp test in the coroutines subdirectory will not include the
-fcoroutines-ts flag, which is added in the local lit config, because
the %{compile} substitution is created long before we add -fcoroutines-ts
to the compiler flags (in the lit.local.cfg for coroutines).