[HLSL] Implement indexed packing of semantic signatures

This issue tracks the implementation of the indexed signature packing algorithm. This should be made available by a function that will take a mutable arrayref of signature elements and update the start row and start col so the elements are packed into a grid.

This is used in the packing of `Target` semantic signatures of pixel shaders.

The indexed packing algorithm places each semantic at the row of it's semantic index and there is no co-packing of elements.

For more comprehensive references:
- [Semantics Overview](https://github.com/llvm/wg-hlsl/blob/main/proposals/0040-semantics-overview.md#signature-packing-constraints)
- [DXIL Spec](https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/DXIL.rst#signature-packing)

AC:
- [ ] A function for the packing algorithm is defined in `llvm/Frontend/HLSL/SemanticSignatures.h`
- [ ] Unit testing of the packing is defined in `llvm/unittests/Frontend/HLSLSemanticSignaturePackingTest.cpp`