[SourceKit] Re-enable cancellation of non-completion requests

This enables the ability to cancel requests, which aren’t code completion requests, again.

Previous crashes in SILGen are prevented by disabling cancellation during the SIL stages. Instead, we add dedicated cancellation checkpoints before and after SIL.

rdar://98390926

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -1621,6 +1621,10 @@ static void computeDiagnostics(
       auto Diagnostics = DiagConsumer.getDiagnosticsForBuffer(BufferID);
       Receiver(RequestResult<DiagnosticsResult>::fromResult(Diagnostics));
     }
+
+    void cancelled() override {
+      Receiver(RequestResult<DiagnosticsResult>::cancelled());
+    }
   };
 
   auto Consumer = std::make_shared<DiagnosticsConsumer>(std::move(Receiver));