Implement the flatten attribute.

This is a GNU attribute that causes calls within the attributed function
to be inlined where possible. It is implemented by giving such calls the
alwaysinline attribute.

Differential Revision: http://reviews.llvm.org/D3816

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209217 91177308-0d34-0410-b5e6-96231b3b80d8