[flang][driver] Disable Clang options in Flang

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.

Differential Revision: https://reviews.llvm.org/D158307

clang/lib/Driver/Driver.cpp

@@ -6495,7 +6495,7 @@ Driver::getOptionVisibilityMask(bool UseDriverMode) const {
   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);
 }