Implement dependent layouts for raw types

Update raw_layout.swift

Allow for concrete specializations of raw layout

Make test platform agnostic

stdlib/public/runtime/Metadata.cpp

@@ -2905,6 +2905,35 @@ metadata:
   }
 }
 
+/// Initialize the value witness table for a @_rawLayout struct.
+SWIFT_RUNTIME_EXPORT
+void swift::swift_initRawStructMetadata(StructMetadata *structType,
+                                        StructLayoutFlags layoutFlags,
+                                        const TypeLayout *likeTypeLayout,
+                                        size_t count) {
+  auto vwtable = getMutableVWTableForInit(structType, layoutFlags);
+
+  // The existing vwt function entries are all fine to preserve, the only thing
+  // we need to initialize is the actual type layout.
+  auto size = likeTypeLayout->size;
+  auto stride = likeTypeLayout->stride;
+  auto alignMask = likeTypeLayout->flags.getAlignmentMask();
+  auto extraInhabitantCount = likeTypeLayout->extraInhabitantCount;
+
+  // If our count is greater than or equal 0, we're dealing an array like layout.
+  if (count >= 0) {
+    stride *= count;
+    size = stride;
+  }
+
+  vwtable->size = size;
+  vwtable->stride = stride;
+  vwtable->flags = ValueWitnessFlags()
+                    .withAlignmentMask(alignMask)
+                    .withCopyable(false);
+  vwtable->extraInhabitantCount = extraInhabitantCount;
+}
+
 /***************************************************************************/
 /*** Classes ***************************************************************/
 /***************************************************************************/