Better error reporting for expanded code

[scabug]

Currently errors in macro expansions, which don't have explicitly assigned positions, will be printed positionless. This is utterly confusing especially given that macro-generated code only exists in AST form unless printed with -Yreify-debug-*

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6822?orig=1
Reporter: @xeno-by