d: Cache generated import declarations in a hash_map

d: Cache generated import declarations in a hash_map

Originally, these were cached in the front-end AST node field `isym'.
However, this field is due to be removed in the future.

gcc/d/ChangeLog:

	* imports.cc (imported_decls): Define.
	(class ImportVisitor): Add result_ field.
	(ImportVisitor::result): New method.
	(ImportVisitor::visit (Module *)): Store decl to result_.
	(ImportVisitor::visit (Import *)): Likewise.
	(ImportVisitor::visit (AliasDeclaration *)): Don't cache decl in
	front-end AST node.
	(ImportVisitor::visit (OverDeclaration *)): Likewise.
	(ImportVisitor::visit (FuncDeclaration *)): Likewise.
	(ImportVisitor::visit (Declaration *)): Likewise.
	(build_import_decl): Use imported_decls to cache and lookup built
	declarations.