[HLSL] Restrict to supported targets

Someday we would like to support HLSL on a wider range of targets, but
today targeting anything other than `dxil` is likly to cause lots of
headaches. This adds an error and tests to validate that the expected
target is `dxil-?-shadermodel`.

We will continue to do a best effort to ensure the code we write makes
it easy to support other targets (like SPIR-V), but this error will
prevent users from hitting frustrating errors for unsupported cases.

Reviewed By: jcranmer-intel, Anastasia

Differential Revision: https://reviews.llvm.org/D132056