Clarify comment so this doesn't appear to be a C11-only rule.
Win64 ABI shouldn't extend integer type arguments.

Summary:
MSVC doesn't extend integer types smaller than 64bit, so to preserve
binary compatibility, clang shouldn't either.

For example, the following C code built with MSVC:

unsigned test(unsigned v);
unsigned foobar(unsigned short);
int main() { return test(0xffffffff) + foobar(28); }

Produces the following:

  0000000000000004: B9 FF FF FF FF     mov         ecx,0FFFFFFFFh
  0000000000000009: E8 00 00 00 00     call        test
  000000000000000E: 89 44 24 20        mov         dword ptr [rsp+20h],eax
  0000000000000012: 66 B9 1C 00        mov         cx,1Ch
  0000000000000016: E8 00 00 00 00     call        foobar

And as you can see, when setting up the call to foobar, only cx is overwritten.

If foobar is compiled with clang, then the zero extension added by clang means
the rest of the register, which contains garbage, could be used.

For example if foobar is:

unsigned foobar(unsigned short v) {
    return v;
}

Compiled with clang -fomit-frame-pointer -O3 gives the following assembly:

foobar:
  0000000000000000: 89 C8              mov         eax,ecx
  0000000000000002: C3                 ret

And that function would return garbage because the 16 most significant bits of
ecx still contain garbage from the first call.

With this change, the code for that function is now:

foobar:
  0000000000000000: 0F B7 C1           movzx       eax,cx
  0000000000000003: C3                 ret

Reviewers: chapuni, rnk

Reviewed By: rnk

Subscribers: majnemer, cfe-commits

Differential Revision: http://reviews.llvm.org/D4380