Canonicalize generated functions for Core witnesses (#7729)

Use a single `SemIR::Function` per `Core` interface method, whether it's
generated locally or imported. This prevents generating duplicate
functions, which lead to different types when the witness appears in a
`FacetValue` as part of a specific for a class.

We use a `CanonicalValueStore` of `GeneratedFunction` objects that allow
finding an existing FunctionId for a `Generated` special function before
(re-)generating it. Mangling for `Generated` functions is also moved to
use the values from the `GeneratedFunction`'s canonicalization key, so
that we have a consistent source of truth for the unique ID of a
`Generated` function across all files.

New tests are in
`toolchain/check/testdata/impl/custom_witness/destroy.carbon`.

toolchain/check/cpp/impl_lookup.cpp

@@ -164,9 +164,8 @@ static auto BuildCopyWitness(
   }
   // Otherwise it's an enum (or eventually a C struct type). Perform a primitive
   // copy.
-  return BuildPrimitiveCopyWitness(
-      context, loc_id, GetClassScope(context, query_self_const_id),
-      query_self_const_id, query_specific_interface_id);
+  return BuildPrimitiveCopyWitness(context, loc_id, query_self_const_id,
+                                   query_specific_interface_id);
 }
 
 static auto BuildCppUnsafeDerefWitness(