Sema, CodeGen: Ensure that an implicit copy ctor is available more often under...

Sema, CodeGen: Ensure that an implicit copy ctor is available more often under the Microsoft C++ ABI.

This is needed because whether the constructor is deleted can control whether
we pass structs by value directly.

To fix this properly we probably want a more direct way for CodeGen to ask
whether the constructor was deleted.

Fixes PR31049.

Differential Revision: https://reviews.llvm.org/D26822

llvm-svn: 287600