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.