compiler: no nil check needed for closure var dereferences

compiler: no nil check needed for closure var dereferences

    Add the "no nil check needed" annotation to the dereference operations
    created in Parse::enclosing_var_reference (this is safe since the
    closure object is under control of the compiler, and pointer fields in
    it will always be non-nil).

    Reviewed-on: https://go-review.googlesource.com/81795

From-SVN: r255400