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
fd5d0a88dd