[Sema][ObjC] Do not DiagnoseUseOfDecl in LookupMemberExpr

Summary:
Remove the call to DiagnoseUseOfDecl in LookupMemberExpr because:
1. LookupMemberExpr eagerly lookup both getter and setter, reguardless
if they are used or not. It causes wrong diagnostics if you are only
using getter.
2. LookupMemberExpr only diagnoses getter, but not setter.
3. ObjCPropertyOpBuilder already DiagnoseUseOfDecl when building getter
and setter. Doing it again in LookupMemberExpr causes duplicated
diagnostics.

rdar://problem/38479756

Reviewers: erik.pilkington, arphaman, doug.gregor

Reviewed By: arphaman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47280

llvm-svn: 333148