[Sema] Diagnose use of declaration correctly.

Before we skipped that for virtual functions not fully qualified (r81507).
This commit basically reverts this to the older behaviour, which seems
more consistent. We now also correctly consider ill-formed calls to deleted
member functions, which were silently passed before in some cases.
The review contains the whole discussion.

PR:		20268
Differential Revision:	 http://reviews.llvm.org/D11334


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242857 91177308-0d34-0410-b5e6-96231b3b80d8