[OpenMP] Do not use the default pipeline without optimizations

Summary:
A previous patch added the option to use the default pipeline when
perfomring LTO rather than the regular LTO pipeline. This greatly
improved performance regressions we were observing with the LTO
pipeline. However, this should not be used if the user explicitly
disables optimizations as the default pipeline expects some
optimizatoins to be perfomed.

Author: jhuber6

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

@@ -860,14 +860,15 @@ std::unique_ptr<lto::LTO> createLTO(
   // TODO: Handle index-only thin-LTO
   Backend =
       lto::createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
-  Conf.UseDefaultPipeline = true;
 
   Conf.CPU = Arch.str();
   Conf.Options = codegen::InitTargetOptionsFromCodeGenFlags(TheTriple);
 
   Conf.MAttrs = getTargetFeatures(TheTriple);
   Conf.CGOptLevel = getCGOptLevel(OptLevel[1] - '0');
   Conf.OptLevel = OptLevel[1] - '0';
+  if (Conf.OptLevel > 0)
+    Conf.UseDefaultPipeline = true;
   Conf.DefaultTriple = TheTriple.getTriple();
   Conf.DiagHandler = diagnosticHandler;