[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/SwiftLangSupport.cpp

@@ -307,6 +307,11 @@ 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;