[Driver] Move CommonArgs to a location visible by the Frontend Drivers

Update initial patch to use std::optional return values for the parsing
functions. This is a more elegant way of returning just the Values to
the Frontend Drivers.

Author: Cameron McInally

clang/include/clang/Driver/CommonArgs.h

@@ -271,10 +271,9 @@ void handleVectorizeLoopsArgs(const llvm::opt::ArgList &Args,
 void handleVectorizeSLPArgs(const llvm::opt::ArgList &Args,
                             llvm::opt::ArgStringList &CmdArgs);
 
-void ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
-                                   const llvm::opt::ArgList &Args,
-                                   llvm::opt::ArgStringList &CmdArgs,
-                                   bool isCompilerDriver);
+std::optional<StringRef> ParseMPreferVectorWidthOption(
+    clang::DiagnosticsEngine &Diags, const llvm::opt::ArgList &Args,
+    llvm::opt::ArgStringList &CmdArgs, bool isCompilerDriver);
 
 } // end namespace tools
 } // end namespace driver

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -3168,17 +3168,16 @@ void tools::handleInterchangeLoopsArgs(const ArgList &Args,
     CmdArgs.push_back("-floop-interchange");
 }
 
-void tools::ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
-                                          const llvm::opt::ArgList &Args,
-                                          ArgStringList &CmdArgs,
-                                          bool isCompilerDriver) {
+std::optional<StringRef> tools::ParseMPreferVectorWidthOption(
+    clang::DiagnosticsEngine &Diags, const llvm::opt::ArgList &Args,
+    ArgStringList &CmdArgs, bool isCompilerDriver) {
   // If this was invoked by the Compiler Driver, we pass through the option
   // as-is. Otherwise, if this is the Frontend Driver, we want just the value.
   StringRef Out = (isCompilerDriver) ? "-mprefer-vector-width=" : "";
 
   Arg *A = Args.getLastArg(clang::driver::options::OPT_mprefer_vector_width_EQ);
   if (!A)
-    return;
+    return std::nullopt;
 
   StringRef Value = A->getValue();
   unsigned Width;
@@ -3188,9 +3187,9 @@ void tools::ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
   if (Value != "none" && Value.getAsInteger(10, Width)) {
     Diags.Report(clang::diag::err_drv_invalid_value)
         << A->getOption().getName() << Value;
-    return;
+    return std::nullopt;
   }
 
   CmdArgs.push_back(Args.MakeArgString(Out + Value));
-  return;
+  return Value;
 }

flang/lib/Frontend/CompilerInvocation.cpp

@@ -311,11 +311,9 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
     opts.LLVMPassPlugins.push_back(a->getValue());
 
   llvm::opt::ArgStringList result;
-  clang::driver::tools::ParseMPreferVectorWidthOption(
+  opts.PreferVectorWidth = clang::driver::tools::ParseMPreferVectorWidthOption(
       diags, args, result,
-      /*isCompilerDriver=*/false);
-  if (result.size() > 0)
-    opts.PreferVectorWidth = result[0];
+      /*isCompilerDriver=*/false).value_or("");
 
   // -fembed-offload-object option
   for (auto *a :