[DirectX] Get resource information via TargetExtType

Instead of storing an auxilliary structure with the information from the
DXIL resource target extension types duplicated, access the information
that we can via the type itself.

This also means we need to handle some of the target extension types we
haven't fully defined yet, like Texture and CBuffer. For now we make an
educated guess to what those should look like based on llvm/wg-hlsl#76,
and we can update them fairly easily when we've defined them more
thoroughly.

First part of #118400