d: Fix ICE on explicit immutable struct import [PR108877]

d: Fix ICE on explicit immutable struct import [PR108877]

Const and immutable types are built as variants of the type they are
derived from, and TYPE_STUB_DECL is not set for these variants.

	PR d/108877

gcc/d/ChangeLog:

	* imports.cc (ImportVisitor::visit (EnumDeclaration *)): Call
	make_import on TYPE_MAIN_VARIANT.
	(ImportVisitor::visit (AggregateDeclaration *)): Likewise.
	(ImportVisitor::visit (ClassDeclaration *)): Likewise.

gcc/testsuite/ChangeLog:

	* gdc.dg/imports/pr108877a.d: New test.
	* gdc.dg/pr108877.d: New test.

(cherry picked from commit ce1cea3e22f58bbddde017f8a92e59bae8892339)