[Runtime] Only cache completed type metadata for associated type witnesses.

This is the scheme used for metadata caches elsewhere, and it
eliminates the need to check the metadata state along the hot path of
swift_getAssociatedTypeWitness().

Author: DougGregor

stdlib/public/runtime/Metadata.cpp

@@ -3968,7 +3968,8 @@ swift::swift_getAssociatedTypeWitness(MetadataRequest request,
   auto witness = ((const void* const *)wtable)[witnessIndex];
   if (LLVM_LIKELY((uintptr_t(witness) &
          ProtocolRequirementFlags::AssociatedTypeMangledNameBit) == 0)) {
-    return swift_checkMetadataState(request, (const Metadata *)witness);
+    // Cached metadata pointers are always complete.
+    return MetadataResponse{(const Metadata *)witness, MetadataState::Complete};
   }
 
   // Find the mangled name.
@@ -4028,11 +4029,19 @@ swift::swift_getAssociatedTypeWitness(MetadataRequest request,
                mangledName.str().c_str());
   }
 
-  // Update the witness table.
+
   assert((uintptr_t(assocTypeMetadata) &
             ProtocolRequirementFlags::AssociatedTypeMangledNameBit) == 0);
-  reinterpret_cast<const void**>(wtable)[witnessIndex] = assocTypeMetadata;
-  return swift_checkMetadataState(request, assocTypeMetadata);
+
+  // Check the metadata state.
+  auto response = swift_checkMetadataState(request, assocTypeMetadata);
+
+  // If the metadata was completed, record it in the witness table.
+  if (response.State == MetadataState::Complete) {
+    reinterpret_cast<const void**>(wtable)[witnessIndex] = assocTypeMetadata;
+  }
+
+  return response;
 }
 
 /***************************************************************************/