parser: Location of typevars includes the quote

This extends the location attached to type variables to include the
preceding quote. Before:

    1 | let x:(([`A] as 'a)* ([`B] as 'a)) = [`A]
                                       ^

After:

    1 | let x:(([`A] as 'a)* ([`B] as 'a)) = [`A]
                                      ^^

The quote is not included in the AST but is logically part of the ident
as suggested by this error message:

    1 | fun (x : 'a t as 'a) -> ();;
                          ^
    Error: This alias is bound to type "'a t" but is used as an instance of type "'a"
           The type variable "'a" occurs inside "'a t"