Implement the no_split_stack attribute.

This is a GNU attribute that allows split stacks to be turned off on a
per-function basis.

Differential Revision: http://reviews.llvm.org/D3817

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209167 91177308-0d34-0410-b5e6-96231b3b80d8