[HLSL][RootSignature] Metadata generation of StaticSampler (#142642)

Implements metadata generation of a Root Signature from its in-memory
representation. It follows the same style as:
https://github.com/llvm/llvm-project/pull/139633.

This pr handles `StaticSamplers`. It also handles converting the else-if
chain into a `std::visit` to allow for future compiler warnings when
adding additional `RootElement` variants.

The metadata follows the format described
[here](https://github.com/llvm/wg-hlsl/blob/main/proposals/0002-root-signature-in-clang.md#metadata-schema).

- Implement `BuildStaticSampler` into HLSLRootSignature.h
- Add sample testcases demonstrating functionality

Note: there is no validation of metadata nodes as the
`llvm::hlsl::rootsig::RootElement` that generates it will have already
been validated.

Resolves https://github.com/llvm/llvm-project/issues/126586

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