gccrs: Experiment with adding an error code to an error

gcc/rust/ChangeLog:

	* rust-diagnostics.cc (rust_error_at): New overload.
	* rust-diagnostics.h (struct ErrorCode): New struct.
	(rust_error_at): New.
	(rust_be_error_at): Likewise.
	* rust-gcc-diagnostics.cc (class rust_error_code_rule): New class.
	(rust_be_error_at): New function.
	* typecheck/rust-casts.cc (TypeCastRules::emit_cast_error): Emit E0054
	when reporting invalid cast error.

gcc/testsuite/ChangeLog:

	* rust/compile/bad_as_bool_char.rs: Add error code to testcase.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>