[libc++] Provide a method for adding compiler flags in lit.local.cfg files

That way, local lit configuration files don't have to worry about
deep-copying the compiler instance of the test format, which is
arguably an implementation detail.

We pass the config to this method even though it is not used by the
current test format because this allows replacing the current test
format by other test formats that would require the config to add
new compile flags.