[SourceKit] Allow explicit cancellation of requests with a cancellation token

The key changes here are
- To keep track of cancellation tokens for all `ScheduledConsumer`s in `SwiftASTManager`
- Generate unique request handles for all incoming requests (`create_request_handle `), use these request handles as cancellation tokens and return them from the `sourcekitd_send_request` methods
- Implement cancellation with `sourcekitd_cancel_request` as the entry point and `SwiftASTManager::cancelASTConsumer` as the termination point

Everything else is just plumbing the cancellation token through the various abstraction layers.

rdar://83391505

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -1407,7 +1407,8 @@ void SwiftLangSupport::findRenameRanges(
 
 void SwiftLangSupport::findLocalRenameRanges(
     StringRef Filename, unsigned Line, unsigned Column, unsigned Length,
-    ArrayRef<const char *> Args, CategorizedRenameRangesReceiver Receiver) {
+    ArrayRef<const char *> Args, SourceKitCancellationToken CancellationToken,
+    CategorizedRenameRangesReceiver Receiver) {
   std::string Error;
   SwiftInvocationRef Invok = ASTMgr->getInvocation(Args, Filename, Error);
   if (!Invok) {
@@ -1447,6 +1448,7 @@ void SwiftLangSupport::findLocalRenameRanges(
   /// don't use 'OncePerASTToken'.
   static const char OncePerASTToken = 0;
   getASTManager()->processASTAsync(Invok, ASTConsumer, &OncePerASTToken,
+                                   CancellationToken,
                                    llvm::vfs::getRealFileSystem());
 }