Prevent link warnings due to -Wframe-larger-than=

Although this is nominally a -W option, we actually handle it in the driver
exactly as an f-group flag that's translated directly to -mllvm.

That means f_Group (and unintuitively, not W_Group) has the semantics we want
to make it behave like a standard warning flag: no automatic forwarding, no
warning for link invocations and compile-only.

Silences diagnostics like:

  [691/1545] Linking CXX executable bin/llvm-diff
  clang-3.5: warning: argument unused during compilation: '-Wframe-larger-than=2048'

(Hopefully we can move towards handling these in the frontend but that'll
require some infrastructure work.)