[HLSL][SPIRV] Clang should run spirv-val if available

The Clang driver adds a step to run the DXIL validator if `dxv` is available, we should add a similar capability for running the SPIRV validator if `spirv-val` is on the path.

This will help us catch a great many issues that we're currently not finding until we try to actually load and execute a compiled shader.

We should also ensure that the -Vd dxc compatible flag disables this as expected.