gccrs: Add support for binding const generic values to paths

[philberty]

Const generics bind values which can be accessed like a normal path but the difference is that they can be true expression values not just type paths. This patch adds support to resolving a method inference which passes a generic value into the method and fixes some missed bugs along the way. The tricky part was that there is a case where in the return position of a method returning a const param type vs the type of the method there is a special case in the unify rules so that we unify the specified type of the const param type not the const param itself.

gcc/rust/ChangeLog:

* backend/rust-compile-resolve-path.cc: handle const param values
* typecheck/rust-hir-type-check-item.cc: generate const infer vars when required
* typecheck/rust-type-util.cc (unify_site_and): handle a null param cleanup
* typecheck/rust-tyty-util.cc (TyVar::get_implicit_const_infer_var): helper interface
* typecheck/rust-tyty-util.h: update header prototypes
* typecheck/rust-tyty.cc (BaseType::is_concrete): correctly handle const types
(ConstParamType::get_name): emit the specified type
(ConstParamType::is_equal): fix recursion loop
* typecheck/rust-unify.cc (UnifyRules::go): const infer vars need cleanup too
* typecheck/rust-unify.h: support base generics

gcc/testsuite/ChangeLog:

* rust/execute/torture/const-generics-2.rs: New test.