ms-inline-asm: Correctly mark MS inline ASM labels as used

Summary: This fixes PR21155.

Test Plan: The patch includes a test.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5619

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219322 91177308-0d34-0410-b5e6-96231b3b80d8