Dump PrettyStackTrace on Ctrl-T in a frontend job (#26234)

It'll dump it for all the frontend jobs spawned from a driver, too,
but then the driver will buffer the response, so it's not that useful.

Requires LLVM r365911.

lib/FrontendTool/FrontendTool.cpp

@@ -1607,6 +1607,7 @@ int swift::performFrontend(ArrayRef<const char *> Args,
                            const char *Argv0, void *MainAddr,
                            FrontendObserver *observer) {
   INITIALIZE_LLVM();
+  llvm::EnablePrettyStackTraceOnSigInfoForThisThread();
 
   PrintingDiagnosticConsumer PDC;