[Sema] Fix parsing of anonymous union in language linkage specification

C++17 [dcl.link]p4:
A linkage specification does not establish a scope.

C++17 [class.union.anon]p2:
Namespace level anonymous unions shall be declared static.

Differential Revision: https://reviews.llvm.org/D45884

rdar://problem/37545925

llvm-svn: 334062