[clang] [HLSL][RootSignature] Implement Lexing of DescriptorTables (PR #122981)

[Finn Plummer via cfe-commits]

inbelic wrote:

HLSL Build failure is because there is an assertion that DIAG_SIZE_LEX is not big enough (insufficient). I did add some new diag errors under the Lex category so it seems the threshold is passed on some environments.

>From [here](https://github.com/llvm/llvm-project/blame/617278e7b0c937fccbf7d67d14f053c3409bc33f/clang/include/clang/Basic/DiagnosticIDs.h#L40), it seems that we could, and there is trend to, bump this limit when needed. IIUC, I should create a separate pr to bump this.

https://github.com/llvm/llvm-project/pull/122981