Change x86_fastcallcc to require inreg markers. This allows it to known

Change x86_fastcallcc to require inreg markers. This allows it to known
the difference from "int x" (which should go in registers and
"struct y {int x;}" (which should not).

Clang will be updated in the next patches.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166536 91177308-0d34-0410-b5e6-96231b3b80d8