Merging r340101:

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r340101 | rnk | 2018-08-17 15:11:31 -0700 (Fri, 17 Aug 2018) | 14 lines

Don't warn on returning the address of a label from a statement expression

Summary:
There isn't anything inherently wrong with returning a label from a
statement expression. In practice, the Linux kernel uses this pattern to
materialize PCs.

Fixes PR38569

Reviewers: niravd, rsmith, nickdesaulniers

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50805
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@340103 91177308-0d34-0410-b5e6-96231b3b80d8