When we're performing name lookup for a tag, we still allow ourselves

When we're performing name lookup for a tag, we still allow ourselves
to see hidden declarations because every tag lookup is effectively a
redeclaration lookup. For example, image that

  struct foo;

is declared in a submodule that is known but hasn't been imported. If
someone later writes

  struct foo *foo_p;

then "struct foo" is either a reference or a redeclaration. To keep
the redeclaration chains sound, we treat it like a redeclaration for
name-lookup purposes.

llvm-svn: 147588