MS ABI: Fix __fastcall methods that return structs

The sret paramater consumes the register after the implicit 'this'
parameter, as with other calling conventions.

Fixes PR20278, which turned out to be very easy.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212669 91177308-0d34-0410-b5e6-96231b3b80d8
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.)

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212670 91177308-0d34-0410-b5e6-96231b3b80d8