This makes it possible to be used in all modes, instead of just when
-fms-extensions is enabled. Also moves the -fms-extensions-exclusive
traits into their own file so we can check the others aren't dependent
on this flag.
This is information that the compiler already has, and should be exposed
so that the library doesn't need to reimplement the exact same
This was originally a part of D116280.
Depends on D135177.