[LTO] Handle Task=-1 passed to addSaveTemps

[tejohnson]

Summary

This change is necessary for D46464, which will pass -1 as the Task
ID for distributed backends, so that the save temps files don't end
up with "4294967295" in their path. For distributed back ends, when -1
is passed, don't append any Task ID.

An existing test (tools/clang/test/CodeGen/thinlto_backend.ll) will
fail without this change after D46464.

[pcc]

LGTM

lib/LTO/LTOBackend.cpp

Hook = [=](unsigned Task, const Module &M) {
       if (LinkerHook && !LinkerHook(Task, M))
         return false;
 
       std::string PathPrefix;
       // If this is the combined module (not a ThinLTO backend compile) or the
       // user hasn't requested using the input module's path, emit to a file
       // named from the provided OutputFileName with the Task ID appended.
       if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
-        PathPrefix = OutputFileName + utostr(Task);
+        PathPrefix = OutputFileName;
+        if (Task != (unsigned)-1)
+          PathPrefix += utostr(Task) + ".";
       } else
-        PathPrefix = M.getModuleIdentifier();
-      std::string Path = PathPrefix + "." + PathSuffix + ".bc";
+        PathPrefix = M.getModuleIdentifier() + ".";
+      std::string Path = PathPrefix + PathSuffix + ".bc";
       std::error_code EC;
       raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
       // Because -save-temps is a debugging feature, we report the error
       // directly and exit.
       if (EC)
         reportOpenError(Path, EC.message());
       WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false);
       return true;