Add support for #pragma nounroll.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213885 91177308-0d34-0410-b5e6-96231b3b80d8
Preserve libclang ABI compatibility with  the 3.5 release

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213886 91177308-0d34-0410-b5e6-96231b3b80d8