[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.

unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

@@ -113,7 +113,8 @@ class CursorInfoTest : public ::testing::Test {
   NullEditorConsumer Consumer;
 
 public:
-  LangSupport &getLang() { return Ctx.getSwiftLangSupport(); }
+  SourceKit::Context &getContext() { return Ctx; }
+  LangSupport &getLang() { return getContext().getSwiftLangSupport(); }
 
   void SetUp() override {
     llvm::InitializeAllTargets();
@@ -496,7 +497,7 @@ TEST_F(CursorInfoTest, CursorInfoCancellation) {
         CursorInfoSema.signal();
       });
 
-  getLang().cancelRequest(CancellationToken);
+  getContext().getRequestTracker()->cancel(CancellationToken);
 
   bool expired = CursorInfoSema.wait(30 * 1000);
   if (expired)