[TableGen] Allow references to class template arguments in defvar

We can't refer to template arguments for defvar statements in class
definitions, or it will report some errors like:

```
error: Variable not defined: 'xxx'.
```

The key point here is we used to pass nullptr to `ParseValue` in
`ParseDefvar`. As a result, we can't refer to template arguments
since `CurRec` is nullptr in `ParseIDValue`.

So we add an argument `CurRec` to `ParseDefvar` and provide it
when parsing defvar statements in class definitions.

Reviewed By: tra, simon_tatham

Differential Revision: https://reviews.llvm.org/D148197