gccrs: const generics: Forbid default values in Functions, Traits and Impls

gccrs: const generics: Forbid default values in Functions, Traits and Impls

gcc/rust/ChangeLog:

	* checks/errors/rust-const-checker.cc (ConstChecker::ctx_to_str): Allow
	getting an error string from a specific constant context.
	(ConstChecker::ctx_allows_default): New function, check if a context
	allows default values for Const generics.
	(ConstChecker::visit): Call into `ctx_allows_default`.
	* checks/errors/rust-const-checker.h: Declare `ctx_allows_default`.

gcc/testsuite/ChangeLog:

	* rust/compile/const_generics_8.rs: New test.