Bring our handling of -Wframe-larger-than more in line with GCC.

Support -Wno-frame-larger-than (with no =) and make it properly
interoperate with -Wframe-larger-than. Reject -Wframe-larger-than with
no argument.

We continue to support Clang's old spelling, -Wframe-larger-than=, for
compatibility with existing users of that facility.

In passing, stop the driver from accepting and ignoring
-fwarn-stack-size and make it a cc1-only flag as intended.