Incorrect underlined position in error message when line contains tabs

```scala
$ echo "\tclass Foo extends Bar" > tab.scala
$ dotc tab.scala
-- [E006] Unbound Identifier Error: try/tab.scala ------------------------------
1 |     class Foo extends Bar
  |                  ^^^
  |                  not found: type Bar
$ scalac tab.scala
tab.scala:1: error: not found: type Bar
        class Foo extends Bar
                          ^
```