Re-apply r216491 (Win64 ABI shouldn't extend integer type arguments.)

This time though, preserve the extension for bool types since that's compatible
with what MSVC expects.

See http://reviews.llvm.org/D4380


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216507 91177308-0d34-0410-b5e6-96231b3b80d8