[HLSL] Add support for user semantics (#153424)
This commit adds support for HLSL input semantics. User semantics are
all semantics not starting with `SV_`.
Those semantics ends up with a Location assignment in SPIR-V.
Note: user semantics means Location, but the opposite is not true.
Depending on the stage, some system semantics can rely on a Location
index. This is not implemented in this PR.
