FrozenHeap always interns strings

Summary:
During starlark execution, strings can be created and destroyed, and the normal `Heap` should not be interning strings if it can help it.

When we freeze heaps, though, strings no longer change. If we intern them (into their respective frozen heaps), we can gain some retained memory back.

Reviewed By: stepancheg

Differential Revision: D66388344

fbshipit-source-id: e2f5bcc4a6669649e99cbc9099e8f6a8925988dc

starlark/src/values/layout/heap/heap_type.rs

@@ -311,10 +311,13 @@ impl FrozenHeap {
         }
     }
 
-    /// Allocate a string on this heap. Be careful about the warnings
-    /// around [`FrozenValue`].
+    /// Allocate a string on this heap. Be careful about the warnings around
+    /// [`FrozenValue`].
+    ///
+    /// Since the heap is frozen, we always prefer to intern the string in order
+    /// to deduplicate it and save some memory.
     pub fn alloc_str(&self, x: &str) -> FrozenStringValue {
-        self.alloc_str_impl(x, StarlarkStr::UNINIT_HASH)
+        self.alloc_str_intern(x)
     }
 
     /// Intern string.