[flang][driver] Partial revert of D157837

This is a partial revert of https://reviews.llvm.org/D157837 - it turns
out that the LLVM test suite needs to be updated first not to use any of
the unsupported Flang options:
  * https://github.com/llvm/llvm-test-suite
Sample errors:
```
flang-new: error: unknown argument: '-fbounds-check'
flang-new: error: unknown argument: '-fcheck=all'
flang-new: error: unknown argument: '-fcheck-array-temporaries'
```

Once the test suite is updated, we can restore the reverted setting.

Broken bot:
  * https://lab.llvm.org/buildbot/#/builders/197/builds/9001

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

clang/lib/Driver/Driver.cpp

@@ -6494,8 +6494,8 @@ Driver::getOptionVisibilityMask(bool UseDriverMode) const {
     return llvm::opt::Visibility(options::CLOption);
   if (IsDXCMode())
     return llvm::opt::Visibility(options::DXCOption);
-  if (IsFlangMode()) {
-    return llvm::opt::Visibility(options::FlangOption);
+  if (IsFlangMode())  {
+    return llvm::opt::Visibility(options::ClangOption | options::FlangOption);
   }
   return llvm::opt::Visibility(options::ClangOption);
 }