gccrs: Fix HIR::ClosureExpr copying

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-expr.cc
	(ClosureParam::ClosureParam): Set locus.
	(ClosureParam::operator=): Likewise.
	(ClosureExpr::ClosureExpr): Likewise.
	(ClosureExpr::operator=): Likewise.
	* typecheck/rust-hir-type-check-expr.cc
	(TypeCheckExpr::visit (ClosureExpr)): Use
	Mappings::get_lang_item.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4740.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>