Fix a data race in the conformsToProtocol method.

This bug was reported by TSan.

Author: AnnaZaks

stdlib/public/runtime/ProtocolConformance.cpp

@@ -560,10 +560,9 @@ swift::swift_conformsToProtocol(const Metadata *type,
       return FoundConformance.first;
   }
 
-  unsigned failedGeneration = ConformanceCacheGeneration;
-
   // If we didn't have an up-to-date cache entry, scan the conformance records.
   C.SectionsToScanLock.lock();
+  unsigned failedGeneration = ConformanceCacheGeneration;
 
   // If we have no new information to pull in (and nobody else pulled in
   // new information while we waited on the lock), we're done.