gccrs: Support Closure calls as generic trait bounds

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Refactor checking of closures.
	(CompileExpr::generate_possible_fn_trait_call): New function.
	* backend/rust-compile-expr.h: Declare `generate_possible_fn_trait_call`.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/closure2.rs: New test.