[HLSL][RootSignature] Audit RootSignatureParser diagnostic production

[inbelic]

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 and here.

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: #147799