[HLSL] Implement resource initialization by constructors

1. Add resource constructor for explicit binding to `HLSLExternalSemaSource`.
2. Replace current resource initialization function (in Codegen) with init expression that calls the constructor (in Sema).
3. Update default resource constructor to initialize resource handle to `poison` (needs to happen after # 1, otherwise the initialized handle will get overridden).

Design proposal: llvm/wg-hlsl#197
