[clang][Driver] Expose -fexperimental-relative-c++-abi-vtables flag

Initially, this flag was meant to only be used through cc1 and not directly
through the clang driver. However, we accidentally ended up using this flag
as a driver flag already for selecting multilibs within the fuchsia toolchain.
We're currently in an awkward state where it's only accepted as a driver flag
when targeting Fuchsia, and all other instances it can only be added via
-Xclang. Since we're ready to use this in Fuchsia, we can just expose this to
the driver for simplicity.

Differential Revision: https://reviews.llvm.org/D98375