[SourceKit] Add a request tracker that manages cancellaiton

Previously, `SwiftASTManager` and `SlowRequestSimulator` maintained their own list of in-progress cancellation tokens. With code completion cancellation coming up, there would need to be yet another place to track in-progress requests, so let’s centralize it.

While at it, also support cancelling requests before they are scheduled, eliminating the need for a `sleep` in a test case.

The current implementaiton leaks tiny amounts of memory if a request is cancelled after if finishes. I think this is fine because it is a pretty nieche case and the leaked memory is pretty small (a `std::map` entry pointing to a `std::function` + `bool`). Alternatively, we could require the client to always dispose of the cancellation token manually.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -273,7 +273,7 @@ configureCompletionInstance(std::shared_ptr<CompletionInstance> CompletionInst,
 
 SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
     : NotificationCtr(SKCtx.getNotificationCenter()),
-      CCCache(new SwiftCompletionCache) {
+      ReqTracker(SKCtx.getRequestTracker()), CCCache(new SwiftCompletionCache) {
   llvm::SmallString<128> LibPath(SKCtx.getRuntimeLibPath());
   llvm::sys::path::append(LibPath, "swift");
   RuntimeResourcePath = std::string(LibPath.str());
@@ -282,7 +282,7 @@ SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
   Stats = std::make_shared<SwiftStatistics>();
   EditorDocuments = std::make_shared<SwiftEditorDocumentFileMap>();
   ASTMgr = std::make_shared<SwiftASTManager>(
-      EditorDocuments, SKCtx.getGlobalConfiguration(), Stats,
+      EditorDocuments, SKCtx.getGlobalConfiguration(), Stats, ReqTracker,
       RuntimeResourcePath, DiagnosticDocumentationPath);
 
   CompletionInst = std::make_shared<CompletionInstance>();
@@ -307,11 +307,6 @@ void SwiftLangSupport::dependencyUpdated() {
   CompletionInst->markCachedCompilerInstanceShouldBeInvalidated();
 }
 
-void SwiftLangSupport::cancelRequest(
-    SourceKitCancellationToken CancellationToken) {
-  getASTManager()->cancelASTConsumer(CancellationToken);
-}
-
 UIdent SwiftLangSupport::getUIDForDeclLanguage(const swift::Decl *D) {
   if (D->hasClangNode())
     return KindObjC;