some errors (?) in the JPQL grammar

While [working on the Query spec](https://github.com/jakartaee/query/issues/24), I ran into the following things:

```
state_valued_path_expression ::=
state_field_path_expression | *general_identification_variable*
```

How can an identification variable can't represent a basic value?

```
type_discriminator ::=
TYPE(general_identification_variable |
single_valued_object_path_expression |
*input_parameter*)
```

What was input_parameter doing there?

```
single_valued_path_expression ::=
qualified_identification_variable |
**TREAT(qualified_identification_variable AS subtype)** |
state_field_path_expression |
single_valued_object_path_expression
```

What could possibly have been the intention of allowing `TREAT(ENTRY(...))`, `TREAT(KEY(...))`, `TREAT(VALUE(...))` there, in a location where `TREAT()` is not even allowed for a regular `identification_variable`.

I think these things are so clearly wrong that we might as well fix them here too.

Start with the JPQL grammar productions quoted in the issue and compare their allowed alternatives with the surrounding grammar specification. Check linked pull request #753 to understand the work already underway; done means the reported grammar inconsistencies are resolved in the specification.

Written by the indexing model from the issue text.