[Runtime] In debug builds, dirty metadata records.

To facilitate debugging metadata records which are not properly
initialized, upon allocation fill them with a regular byte pattern
(0xAA) so that on subsequent inspection it is obvious if part of the
record is not initialized.

Author: nate-chandler

stdlib/public/runtime/Metadata.cpp

@@ -663,6 +663,11 @@ swift::swift_allocateGenericValueMetadata(const ValueTypeDescriptor *description
 
   auto bytes = (char*) cache.getAllocator().Allocate(totalSize, alignof(void*));
 
+#ifndef NDEBUG
+  // Fill the metadata record with garbage.
+  memset(bytes, 0xAA, totalSize);
+#endif
+
   auto addressPoint = bytes + sizeof(ValueMetadata::HeaderType);
   auto metadata = reinterpret_cast<ValueMetadata *>(addressPoint);