[SourceKit] Hold CompletionInstance with std::shared_ptr

So we can pin it on the threads.

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -172,7 +172,10 @@ static bool swiftCodeCompleteImpl(
     Error = "no input filenames specified";
     return false;
   }
-  CompilerInstance *CI = Lang.getCompletionInstance().getCompilerInstance(
+
+  // Pin completion instance.
+  auto CompletionInst =  Lang.getCompletionInstance();
+  CompilerInstance *CI = CompletionInst->getCompilerInstance(
       Invocation, FileSystem, newBuffer.get(), Offset, Error, &CIDiags);
   if (!CI)
     return false;