gccrs: diagnostics: Added non-const `rich_location *` function

gccrs: diagnostics: Added non-const `rich_location *` function

Currently, gccrs using the const `rich_location &`, and
then using the cast to convert it to the functions which
was used to emit the errors, having the overloaded is
good, similiar to other frontends.

gcc/rust/ChangeLog:

	* rust-diagnostics.cc (rust_be_error_at): Added overload function.
	(rust_error_at): non-const `rich_location *` function.
	* rust-diagnostics.h (rust_error_at): Implementation of overloaded
	function.
	(rust_be_error_at): likewise.
	* rust_error_codes.def: Added GNU license.

Signed-off-by: Muhammad Mahad <mahadtxt@gmail.com>