Merging r367661:

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r367661 | hans | 2019-08-02 09:51:41 +0200 (Fri, 02 Aug 2019) | 5 lines

Don't try emitting dllexported explicitly defaulted non-trivial ctors twice during explicit template instantiation definition (PR42857)

Trying to emit the definition twice triggers an assert.

Differential revision: https://reviews.llvm.org/D65579
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llvm-svn: 367805