-fms-extensions: Alias _intNN to __intNN

Fixes build for SPEC 2000 CPU. MSVC disables these aliases under /Za,
which enables stricter standards compliance. We don't currently have any
way to disable them.

Patch by Kevin Smith!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216270 91177308-0d34-0410-b5e6-96231b3b80d8