Merging r282989:

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r282989 | rtrieu | 2016-09-30 17:15:24 -0700 (Fri, 30 Sep 2016) | 10 lines

Fix crash when emitting error.

With templated classes, is possible to not be able to determine is a member
function is a special member function before the class is instantiated.  Only
these special member functions can be defaulted.  In some cases, knowing
whether a function is a special member function can't be determined until
instantiation, so an uninstantiated function could possibly be defaulted too.
Add a case to the error diagnostic when the function marked with a default is
not known to be a special member function.

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llvm-svn: 286922