gccrs: Closure support at CallExpr

gccrs: Closure support at CallExpr

gcc/rust/ChangeLog:

	* backend/rust-compile-context.h: Add new functions: `insert_closure_decl` and
	`lookup_closure_decl`.
	* backend/rust-compile-expr.cc (CompileExpr::visit): Start compiling Closures properly.
	(CompileExpr::generate_closure_function): New function.
	(CompileExpr::generate_closure_fntype): Likewise.
	* backend/rust-compile-expr.h: Declare `generate_closure_function` and
	`generate_closure_fntype`.
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): Visit closure types properly.
	* backend/rust-mangle.cc (legacy_mangle_name): Add support for closures.
	* backend/rust-tree.h (RS_CLOSURE_FLAG): Add new tree macro.
	(RS_CLOSURE_TYPE_P): And checking for it on tree nodes.
	* typecheck/rust-tyty.cc (ClosureType::is_equal): Add implementation.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/closure1.rs: New test.