[SourceKit] Bug fixes in 'compile'

* Diagnostics consumer wasn't attached in new compilation
* The reuse flag check was flipped

Author: rintaro

lib/IDE/CompileInstance.cpp

@@ -294,7 +294,7 @@ void CompileInstance::performSema(
   if (CI && ArgsHash == CachedArgHash &&
       CachedReuseCount < Opts.MaxASTReuseCount) {
     CI->getASTContext().CancellationFlag = CancellationFlag;
-    if (performCachedSemaIfPossible(DiagC)) {
+    if (!performCachedSemaIfPossible(DiagC)) {
       // If we compileted cacehd Sema operation. We're done.
       ++CachedReuseCount;
       return;
@@ -312,6 +312,9 @@ void CompileInstance::performSema(
     return;
   }
 
+  CI->addDiagnosticConsumer(DiagC);
+  SWIFT_DEFER { CI->removeDiagnosticConsumer(DiagC); };
+
   // CI is potentially reusable.
   CachedArgHash = ArgsHash;