[flang][driver][Revert] Reverts f18 to allow options passed to -W

[arnamoy10]

Summary

f18 runs compilation all the way to the linking step. However, with the implementation of the -Werror option in the frontend driver, to makes tests shareable between the driver and f18, the code of the later was modified so that it terminates compilation if anything other than error is given for -W.

So a command like the following would fail:

flang   -Wl,-z,muldefs    -march=skylake-avx512 -mfpmath=sse -Ofast -funroll-loops -flto   exchange2.fppized.o  -L/export/users/build_1.1.1/jemalloc/intel64/lib/ -ljemalloc    -o exchange2_r

This patch takes away the modification ONLY from f18, so that the flang-new driver still retains the power to cause compilation termination if anything other that -Werror is supplied for the -W option (until they are supported).

[awarzynski]

These changes LGTM - there's no need to limit f18, but we should continue being strict in flang-new and reject everything that's not supported.

Please, could you shorten the title of the commit? As per https://llvm.org/docs/DeveloperPolicy.html#commit-messages

The title should be concise.

I try to limit myself to 50 chars for the title and 72 chars for the body. This is not always possible, but currently your title is >110 chars wide.

Also, from your summary:

This patch takes away the modification ONLY in f18, so that the driver still (...)

Could you clarify which driver you have in mind here? It's clear to me, but I've been working with it quite a lot. I think that it might be unclear to readers unfamiliar with the state of the drivers in Flang.

flang/test/Driver/werror-wrong.f90

 ! Ensure that only argument -Werror is supported.
+! REQUIRES: new-flang-driver
 
 ! RUN: not %flang_fc1 -fsyntax-only -Wall %s  2>&1 | FileCheck %s --check-prefix=WRONG
 ! RUN: not %flang_fc1 -fsyntax-only -WX %s  2>&1 | FileCheck %s --check-prefix=WRONG
 
 !-----------------------------------------
 ! EXPECTED OUTPUT WITH -W<opt>
 !-----------------------------------------
 ! WRONG: Only `-Werror` is supported currently.

flang/tools/f18/f18.cpp

if (arg.empty()) {
       options.features.Enable(Fortran::common::LanguageFeature::OpenACC);
       predefinitions.emplace_back("_OPENACC", "202011");
     } else if (arg == "-fopenmp") {
       options.features.Enable(Fortran::common::LanguageFeature::OpenMP);
       predefinitions.emplace_back("_OPENMP", "201511");
     } else if (arg.find("-W") != std::string::npos) {
       if (arg == "-Werror")
         driver.warningsAreErrors = true;
-      else {
-        // Only -Werror is supported currently
-        llvm::errs() << "Only `-Werror` is supported currently.\n";
-        return EXIT_FAILURE;
-      }
     } else if (arg == "-ed") {
       options.features.Enable(Fortran::common::LanguageFeature::OldDebugLines);
     } else if (arg == "-E") {
       driver.dumpCookedChars = true;
     } else if (arg == "-fbackslash" || arg == "-fno-backslash") {
       options.features.Enable(
           Fortran::common::LanguageFeature::BackslashEscapes,
           arg == "-fbackslash");