r179334 - Fix the driver logic for recent versions of DragonFly.

[Rafael Espíndola]

> +  bool UseGCC47 = false;
>    const Driver &D = getToolChain().getDriver();
>    ArgStringList CmdArgs;
>
> +  if (llvm::sys::fs::exists("/usr/lib/gcc47", UseGCC47))
> +    UseGCC47 = false;

The same comment I had when the patch was first emailed: If you don't
care about errors, why not use:

/// @brief Simpler version of exists for clients that don't need to
///        differentiate between an error and false.
inline bool exists(const Twine &path) {
  bool result;
  return !exists(path, result) && result;
}


The patch is still *very* under tested.

Cheers,
Rafael