Fix error reporting for invalid private method call

[eric-milles]

Consider the following:

class A {
  private x() {
  }
}
@groovy.transform.CompileStatic
class B extends A {
  private y() {
    x()
  }
}

Method call "x()" is invalid and produces a compiler error. However, the error is associated with line -1 and so it appears at the top of the source unit.

[eric-milles]