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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147588 91177308-0d34-0410-b5e6-96231b3b80d8