[Driver][DXC] Handle -Fo and -Fc flags

This splits the backend and assemble actions for HLSL inputs and
handles the options in GetNamedOutputPath instead of aliasing `-o`.
This also moves how we default to emitting asm to stdout, since doing
this in the HLSL toolchain rather than the driver pollutes how the
clang driver works as well.

When both options are specified we disable collapsing the assemble
action and attempt to generate both outputs. Note that while this
handles the driver aspects, we can't actually run in that mode for now
since -cc1as doesn't understand DXIL as an input yet.

Differential Revision: https://reviews.llvm.org/D157582