Restructure how we finalize VWTs in the runtime to potentially allow asynchronous queries.

I keep finding reasons to want such queries and then deciding that they're
unnecessary.  Let's at least do this much, though.

unittests/runtime/Array.cpp

@@ -20,7 +20,8 @@
 using namespace swift;
 
 namespace swift {
-  void installCommonValueWitnesses(ValueWitnessTable *vwtable);
+  void installCommonValueWitnesses(const TypeLayout &layout,
+                                   ValueWitnessTable *vwtable);
 } // namespace swift
 
 
@@ -33,7 +34,7 @@ static void initialize_pod_witness_table_size_uint32_t_stride_uint64_t(
     .withBitwiseTakable(true)
     .withInlineStorage(true);
   testTable.stride = sizeof(uint64_t);
-  installCommonValueWitnesses(&testTable);
+  installCommonValueWitnesses(*testTable.getTypeLayout(), &testTable);
 }
 
 extern "C" void swift_arrayInitWithCopy(OpaqueValue *dest,