[PATCH] gold, libLTO: Add new flags to support bit set lowering.

[Rafael Espíndola]

> I don't think this should be a problem; there is no user-visible behaviour
> change when using the supported way of enabling control flow integrity.
>
> The only supported way to enable CFI for the user to supply -fsanitize=cfi*
> at link time as well as at compile time. (Admittedly this should be
> documented better.) The LowerBitSets pass only has an effect on the module
> if -fsanitize=cfi* was passed as compile time. And the related change
> http://reviews.llvm.org/D8402 modifies the Clang driver to supply the
> lowerbitsets flag if -fsanitize=cfi* is supplied at link time.

If the pass does nothing if a given flag is not present, why not
always run the pass and avoid the option?