Rework how we support C++ implicit member accesses. If we can resolve an

implicit member access to a specific declaration, go ahead and create
it as a DeclRefExpr or a MemberExpr (with implicit CXXThisExpr base) as
appropriate.  Otherwise, create an UnresolvedMemberExpr or
DependentScopeMemberExpr with a null base expression.

By representing implicit accesses directly in the AST, we get the ability
to correctly delay the decision about whether it's actually an instance
member access or not until resolution is complete.  This permits us
to correctly avoid diagnosing the 'problem' of 'MyType::foo()'
where the relationship to the type isn't really known until instantiation.

llvm-svn: 90266