[CodeCompletion] Handle cases where PersistentParserState is not created

For example, if stdlib is not found, CompilerInstance simply returns
before creating PersistentParserState. In such cases, completion should
just return empty result.

Previously, it caused SourceKit crash.

rdar://problem/58663066

Author: rintaro

lib/IDE/CompletionInstance.cpp

@@ -171,6 +171,8 @@ bool CompletionInstance::performCachedOperaitonIfPossible(
 
   auto &CI = *CachedCI;
 
+  if (!CI.hasPersistentParserState())
+    return false;
   auto &oldState = CI.getPersistentParserState();
   if (!oldState.hasCodeCompletionDelayedDeclState())
     return false;
@@ -321,7 +323,8 @@ bool CompletionInstance::performNewOperation(
   registerIDERequestFunctions(CI.getASTContext().evaluator);
 
   CI.performParseAndResolveImportsOnly();
-  Callback(CI);
+  if (CI.hasPersistentParserState())
+    Callback(CI);
 
   if (DiagC)
     CI.removeDiagnosticConsumer(DiagC);

test/SourceKit/CodeComplete/complete_without_stdlib.swift

@@ -0,0 +1,19 @@
+class Str {
+  var value: Str
+}
+
+// rdar://problem/58663066
+// Test a environment where stdlib is not found.
+// Completion should return zero result.
+
+// RUN: %empty-directory(%t/rsrc)
+// RUN: %empty-directory(%t/sdk)
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=complete -pos=4:1 %s -- %s -resource-dir %t/rsrc -sdk %t/sdk | %FileCheck %s
+// RUN: %sourcekitd-test \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=4:1 %s -- %s -resource-dir %t/rsrc -sdk %t/sdk == \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=4:1 %s -- %s -resource-dir %t/rsrc -sdk %t/sdk | %FileCheck %s
+
+// CHECK: key.results: [
+// CHECK-NOT: key.description: