[modules] Remove IRGen special case for emitting implicit special members if
they're somehow missing a body. Looks like this was left behind when the loop
was generalized, and it's not been problematic before because without modules,
a used, implicit special member function declaration must be a definition.

This was resulting in us trying to emit a constructor declaration rather than
a definition, and producing a constructor missing its member initializers.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214473 91177308-0d34-0410-b5e6-96231b3b80d8