[flang][driver] Disable Clang options in Flang

[awarzynski]

Summary

Restore the desired setting that was reverted in

https://reviews.llvm.org/D158289.

This is to be merged once Fortran tests in llvm-test-suite are updated
accordingly:

• https://reviews.llvm.org/D158308

[DavidSpickett]

For reasons I won't get into, buildbots don't show what llvm-test-suite revision they are using but we can be sure by now that the other change has gone through. Fingers crossed for this one.

clang/lib/Driver/Driver.cpp

 Driver::getOptionVisibilityMask(bool UseDriverMode) const {
   if (!UseDriverMode)
     return llvm::opt::Visibility(options::ClangOption);
   if (IsCLMode())
     return llvm::opt::Visibility(options::CLOption);
   if (IsDXCMode())
     return llvm::opt::Visibility(options::DXCOption);
   if (IsFlangMode())  {
-    return llvm::opt::Visibility(options::ClangOption | options::FlangOption);
+    return llvm::opt::Visibility(options::FlangOption);
   }
   return llvm::opt::Visibility(options::ClangOption);
 }
 
 const char *Driver::getExecutableForDriverMode(DriverMode Mode) {
   switch (Mode) {
   case GCCMode:
     return "clang";