Add a comment about type-uniquing ObjC types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205277 91177308-0d34-0410-b5e6-96231b3b80d8
LTO type uniquing: store the Decl field of a DIImportedEntity as a DIRef.
No other functionality changes, DIBuilder testcase is included in a paired
CFE commit.

This relaxes the assertion in isScopeRef to also accept subclasses of
DIScope.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205279 91177308-0d34-0410-b5e6-96231b3b80d8