[HLSL] No error reported when entry point cannot be found

Repro: https://hlsl.godbolt.org/z/rxKexeq1e

Start with the HLSL repro at https://hlsl.godbolt.org/z/rxKexeq1e and investigate how the compiler handles a missing entry point. Trace the diagnostic path for this case and compare it with other invalid-entry-point handling. Done means the missing entry point produces an error diagnostic.

Written by the indexing model from the issue text.