Escaping an RST character to correct a Sphinx warning.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213651 91177308-0d34-0410-b5e6-96231b3b80d8
Adding a heading to the loop hint attributes so that attribute documentation can be generated again.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213652 91177308-0d34-0410-b5e6-96231b3b80d8