Permit redeclaration of tags introduced by using decls

This valid construct appears in MSVC headers where it's used to provide a
definition for the '::type_info' compiler builtin type.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@199490 91177308-0d34-0410-b5e6-96231b3b80d8