clang-cl: Don't warn for unused private fields when encountering a late parsed template member

Summary: This fixes PR21235.

Test Plan: Includes an automated test.

Reviewers: hansw

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5718

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219551 91177308-0d34-0410-b5e6-96231b3b80d8