cppgc: Fix GCInfo folding

Actually apply the folding on allocation.

Bug: chromium:1056170
Change-Id: Ief25fac49c5caff40579fd44249bba0ae82f9689
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2756536
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73377}

include/cppgc/allocation.h

@@ -78,7 +78,7 @@ class MakeGarbageCollectedTraitBase
       static_assert(std::is_base_of<CustomSpaceBase, CustomSpace>::value,
                     "Custom space must inherit from CustomSpaceBase.");
       return internal::MakeGarbageCollectedTraitInternal::Allocate(
-          handle, size, internal::GCInfoTrait<T>::Index(),
+          handle, size, internal::GCInfoTrait<U>::Index(),
           CustomSpace::kSpaceIndex);
     }
   };
@@ -88,7 +88,7 @@ class MakeGarbageCollectedTraitBase
     static void* Allocate(AllocationHandle& handle, size_t size) {
       // Default space.
       return internal::MakeGarbageCollectedTraitInternal::Allocate(
-          handle, size, internal::GCInfoTrait<T>::Index());
+          handle, size, internal::GCInfoTrait<U>::Index());
     }
   };