Suppress a number of warnings in no-assert builds (#17721)

* Supress a number of warnings about things used only in asserts

* Re-use a couple of variables instead of supressing the warning

lib/Frontend/FrontendInputsAndOutputs.cpp

@@ -293,6 +293,7 @@ void FrontendInputsAndOutputs::setMainAndSupplementaryOutputs(
     assert(outputFiles.size() == N && "Must have one main output per primary");
     assert(supplementaryOutputs.size() == N &&
            "Must have one set of supplementary outputs per primary");
+    (void)N;
 
     unsigned i = 0;
     for (auto &input : AllInputs) {