[HLSL] Remove unused frontend-generated ID

As @python3kgae pointed out we're going to want to assign these IDs
after optimization so that we can remove unused resrouces. This patch
just removes the unused ID value from the frontend metadata, clang code
generation, and updates associated test cases.

Reviewed By: python3kgae

Differential Revision: https://reviews.llvm.org/D136271