Microsoft has a language extension which allows union members to be
references.  What's more, they use this language extension in their
ATL header files (which come as part of MFC and the Win32 SDK).  This patch implements support for the Microsoft extension, and addresses PR13737.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182936 91177308-0d34-0410-b5e6-96231b3b80d8