[HLSL][RootSignature] Implement parsing of a DescriptorTable with empty clauses (#133302)

- defines the Parser class and an initial set of helper methods to
support consuming tokens. functionality is demonstrated through a simple
empty descriptor table test case
- defines an initial in-memory representation of a DescriptorTable
- implements a test harness that will be used to validate the correct
diagnostics are generated. it will construct a dummy pre-processor with
diagnostics consumer to do so

Implements the first part of
https://github.com/llvm/llvm-project/issues/126569

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