Fixes #1181/#1501. Fixes ArrayIndexOutOfBoundsException: -1

When the compiler reports back the error to CompilationUnit created by Eval#mkUnit, it sometimes returns OffsetPosition whose `source` is set to `NoSourceFile`.

This causes ArrayIndexOutOfBoundsException. The current workaround is to pattern match on the passed in pos and create a new one when the incoming source looks suspicious.
I have not figured out whether this is caused by our macro code or compiler.
There are various build.sbt errors that would cause this behavior:

```scala
libraryDependencies ++= Seq(
  depA
  depB // missing comma
)

lazy val bob = scala.Console println

test ++

run+
```
Merge pull request #1509 from sbt/fix/1501

Fixes #1181/#1501. Fixes ArrayIndexOutOfBoundsException: -1