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
f03bcb7594