[HLSL] Implement resource initialization by constructors

[hekota]

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