Merging r339210:

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r339210 | rsmith | 2018-08-08 02:42:42 +0200 (Wed, 08 Aug 2018) | 2 lines

PR38286: Don't crash when attempting to define a constructor for an
incomplete class template.
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@339236 91177308-0d34-0410-b5e6-96231b3b80d8