ext_vector IRGen. Patch to allow indexing into 
ext_vector_type's 'hi/lo' components when
used as lvalue. rdar://18031917 pr20697


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215991 91177308-0d34-0410-b5e6-96231b3b80d8