[llvm] [HLSL] Add in-memory representation of Semantic Signatures (PR #209907)

[Finn Plummer via llvm-commits]

inbelic wrote:

This pr is the cause of the msan failures detected [here](https://lab.llvm.org/buildbot/#/builders/94/builds/19767) and [here](https://lab.llvm.org/buildbot/#/builders/164/builds/23878). This is resolved in https://github.com/llvm/llvm-project/pull/212901

https://github.com/llvm/llvm-project/pull/209907