d: Fix ICE in in add_stack_var, at cfgexpand.cc:476

The type that triggers the ICE never got completed by the semantic
analysis pass.  Checking for size forces it to be done, or issue a
compile-time error.

	PR d/106555

gcc/d/ChangeLog:

	* d-target.cc (Target::isReturnOnStack): Check for return type size.

gcc/testsuite/ChangeLog:

	* gdc.dg/imports/pr106555.d: New test.
	* gdc.dg/pr106555.d: New test.