Rewrite of our handling of name lookup in C++ member access expressions, e.g.,

  x->Base::f

We no longer try to "enter" the context of the type that "x" points
to. Instead, we drag that object type through the parser and pass it
into the Sema routines that need to know how to perform lookup within
member access expressions.

We now implement most of the crazy name lookup rules in C++
[basic.lookup.classref] for non-templated code, including performing
lookup both in the context of the type referred to by the member
access and in the scope of the member access itself and then detecting
ambiguities when the two lookups collide (p1 and p4; p3 and p7 are
still TODO). This change also corrects our handling of name lookup
within template arguments of template-ids inside the
nested-name-specifier (p6; we used to look into the scope of the
object expression for them) and fixes PR4703.

I have disabled some tests that involve member access expressions
where the object expression has dependent type, because we don't yet
have the ability to describe dependent nested-name-specifiers starting
with an identifier.

llvm-svn: 80843