Fix member refs with using decl + anonymous union.

Make sure we call BuildFieldReferenceExpr with the appropriate decl
when a member of an anonymous union is made public with a using decl.
Also, fix a crash on invalid field access into an anonymous union.

Fixes PR16630.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186367 91177308-0d34-0410-b5e6-96231b3b80d8