tint/utils: Remove non-const accessors on VectorRef

Nothing uses these, and the mutability of these breaks
const-correctness.

Switch functions that used to return `const utils::Vector<T, N>&`
to returning `utils::VectorRef<T>`. Removes the templated size from the
public interface.

Replace all `const utils::VectorRef<T>&` with `utils::Vector<T>`,
there's no point in using yet another level of pointer indirection.

Change-Id: Ib96e3171500606d9afffbb13f40023552a74fffc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113021
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

src/tint/transform/direct_variable_access.cc

@@ -228,7 +228,8 @@ struct DirectVariableAccess::State {
         // will have the pointer parameters replaced with an array of u32s, used to perform the
         // pointer indexing in the variant.
         // Function call pointer arguments are replaced with an array of these dynamic indices.
-        for (auto* decl : utils::Reverse(sem.Module()->DependencyOrderedDeclarations())) {
+        auto decls = sem.Module()->DependencyOrderedDeclarations();
+        for (auto* decl : utils::Reverse(decls)) {
             if (auto* fn = sem.Get<sem::Function>(decl)) {
                 auto* fn_info = FnInfoFor(fn);
                 ProcessFunction(fn, fn_info);