DR407: Rationalize how we handle tags being hidden by typedefs. Even with

DR407, the C++ standard doesn't really say how this should work. Here's what we
do (which is consistent with DR407 as far as I can tell):

 * When performing name lookup for an elaborated-type-specifier, a tag
   declaration hides a typedef declaration that names the same type.
 * When performing any other kind of lookup, a typedef declaration hides
   a tag declaration that names the same type.

In any other case where lookup finds both a typedef and a tag (that is, when
they name different types), the lookup will be ambiguous. If lookup finds a
tag and a typedef that name the same type, and finds anything else, the lookup
will always be ambiguous (even if the other entity would hide the tag, it does
not also hide the typedef).

llvm-svn: 252959