[HLSL][RootSignature] Implement diagnostic for missed comma (#147350)

This pr fixes a bug that allows parameters to be specified without an
intermediate comma.

After this pr, we will correctly produce a diagnostic for (eg):
```
RootFlags(0) CBV(b0)
```

This pr updates the problematic code pattern containing a chain of 'if'
statements to a chain of 'else if' statements, to prevent parsing of an
element before checking for a comma.

This pr also does 2 small updates, while in the region:
1. Simplify the `do` loop that these `if` statements are contained in.
This helps code readability and makes it easier to improve the
diagnostics further
2. Moves the `consumeExpectedToken` function calls to be right after the
`parse.*Params` invocation. This will ensure that the comma or invalid
token error is presented before a "missed mandatory param" diagnostic.

- Updates all occurrences of the if chains with an else-if chain
- Simplifies the surrounding `do` loop to be an easier to understand
`while` loop
- Moves the `consumeExpectedToken` diagnostic right after the loop so
that the missing comma diagnostic is produce before checking for any
missed mandatory arguments
- Adds unit tests for this scenario
- Small fix to the diagnostic of `RootDescriptors` to use their
respective `Token` instead of `RootConstants`

Resolves: https://github.com/llvm/llvm-project/issues/147337