[Driver][HLSL] Improve diagnostics for invalid shader model and stage

This adds more validation that a dxil triple is actually useable when
compiling HLSL.

The OS field of the triple needs to be a versioned shader model.
Later, we should set a default if this is empty and check that the
version is a shader model we can actually handle.

The Environment field of the triple needs to be specified and be a
valid shader stage. I'd like to allow this to be empty and treat it
like library, but allowing that currently crashes in DXIL metadata
handling.

Differential Revision: https://reviews.llvm.org/D159103