tint: Fix copy instead of move of ArrayLengthFromUniform::used_size_indices

In our backends, we std::move this unordered_set into the Results
struct, but because we'd get a const reference to a const object,
std::move would cast this to a const r-value, which binds to const ref,
and thus would end up copying instead of moving. This change just
ensures that we get a non-const ref to a non-const unordered_set where
we do the move.

Bug: tint:1495
Change-Id: I8d1cfe66b8343bae4146fc85fc86d72bac41a1a1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86202
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/tint/transform/array_length_from_uniform.h

@@ -98,7 +98,7 @@ class ArrayLengthFromUniform final
     ~Result() override;
 
     /// Indices into the UBO that are statically used.
-    const std::unordered_set<uint32_t> used_size_indices;
+    std::unordered_set<uint32_t> used_size_indices;
   };
 
   /// @param program the program to inspect

src/tint/transform/transform.h

@@ -88,6 +88,13 @@ class DataMap {
   /// Put()
   template <typename T>
   T const* Get() const {
+    return const_cast<DataMap*>(this)->Get<T>();
+  }
+
+  /// @returns a pointer to the Data placed into the DataMap with a call to
+  /// Put()
+  template <typename T>
+  T* Get() {
     auto it = map_.find(&TypeInfo::Of<T>());
     if (it == map_.end()) {
       return nullptr;