[HLSL] Entry functions require param annotation

HLSL entry function parameters must have parameter annotations. This
allows appropriate intrinsic values to be populated into parameters
during code generation.

This does not handle entry function return values, which will be
handled in a subsequent commit because we don't currently support any
annotations that are valid for function returns.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D131625