hasElaborateAssign -> hasElaborateCopyConstructor.

Corrected check for elaborate copying.
Merge pull request #2437 from yglukhov/patch-1

hasElaborateAssign -> hasElaborateCopyConstructor.
fix Issue 13300 - pure function 'std.array.Appender!(T[]).Appender.ensureAddable' cannot call impure function 'test.T.__fieldPostBlit'