SR-7732: Dynamic casting CFError to Error results in a memory leak (#28686)

* SR-7732: Dynamic casting CFError to Error results in a memory leak

The special handling for casting CFError/NSError to Swift Error
type was using cleanup code that didn't correctly handle this case.
This replaces the cleanup code with a more targeted version.

Fixes: SR-7732
Fixes: rdar://problem/40423061

* Whitespace fixes

* Don't rely on localizable strings to verify test behavior.

I've verified this simplified test still leaks with
the original code and does not leak with the fixed code.

* Don't test against old runtimes that predate this fix

* Explicitly test both NSError and CFError

stdlib/public/runtime/Casting.cpp

@@ -911,7 +911,9 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
                                                         srcDynamicType,
                                                         errorWitness)) {
       *destBoxAddr = reinterpret_cast<SwiftError*>(embedded);
-      maybeDeallocateSource(true);
+      if (shouldDeallocateSource(true, flags)) {
+        srcType->vw_destroy(src);
+      }
       return true;
     }
 #endif