[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.

llvm-svn: 214473