d: Fix ICE in must_pass_in_stack_var_size_or_pad with D enums [PR123411]

d: Fix ICE in must_pass_in_stack_var_size_or_pad with D enums [PR123411]

An `enum : enum A` type caused the already computed underlying type size
of `enum A` to be overwritten with NULL_TREE.  To fix, don't finish the
enum with layout_type unless we're handling the main variant type.

	PR d/123411

gcc/d/ChangeLog:

	* types.cc (TypeVisitor::visit (TypeEnum *)): Only call layout_type on
	the TYPE_MAIN_VARIANT of the enum.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr123411.d: New test.