[SourceKit] Add cancellation points for 'compile' requests

After each stage of the compilation pipeline before the code generation.
Sema, SILGen, SILOptimization, SILLowering, IRGen.

lib/Frontend/Frontend.cpp

@@ -1368,6 +1368,10 @@ bool CompilerInstance::performSILProcessing(SILModule *silModule) {
   return false;
 }
 
+bool CompilerInstance::isCancellationRequested() const {
+  auto flag = getASTContext().CancellationFlag;
+  return flag && flag->load(std::memory_order_relaxed);
+}
 
 const PrimarySpecificPaths &
 CompilerInstance::getPrimarySpecificPathsForWholeModuleOptimizationMode()