[HLSL][RootSignature] Audit `RootSignatureParser` diagnostic production (#147800)

This pr audits the diagnostics produced in `RootSignatureParser`
diagnostics.

First, it has been noted more than once that the current
`diag::err_hlsl_unexpected_end_of_params` is not direct and can be
misleading. For instance,
[here](https://github.com/llvm/llvm-project/pull/147350#discussion_r2193717272)
and
[here](https://github.com/llvm/llvm-project/pull/145827#discussion_r2169406679).

This pr address this by removing this diagnostic and replacing it with a
more direct `diag::err_expected_either`. However, doing so removes the
nuance between the case where it is a missing comma and when it was an
invalid parameter.

Hence, we introduce the `diag::err_hlsl_invalid_token` for the latter
case, which does so in a direct way. Further, we can apply the same
diagnostic to the parsing of parameter values.

As part of this, we see that there was a test gap in testing the
diagnostics produced from `diag::err_expected_after` and for the parsing
of enum/flag values. As such, these are also addressed here to provide
sufficient unit/sema test coverage.

- Removes all uses of `diag::err_hlsl_unexpected_end_of_params` in
`RootSigantureParser`
- Introduce `diag::err_hlsl_invalid_token` to provide a direct
diagnostic
- In each of these cases, replace the diagnostic with either a
`diag::err_hlsl_invalid_token` or `diag::err_expected_either`
accordingly
- Update `HLSLRootSignatureParserTest` to account for diagnostic changes
- Increase test coverage of `HLSLRootSignatureParserTest` for enum/flag
diagnostics
- Increase test coverage of `RootSignatures-err` for enum/flag
diagnostics
- Small fix-up of the `diag::err_expected_after` and add test to
demonstrate usage

Resolves: https://github.com/llvm/llvm-project/issues/147799
5 files changed
tree: 1d2aecacf07830645e97f6bd4437de0a9e912ae8
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-format-ignore
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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