[mlir] Report line number from file rather than split (#150982)

Add convention for lexer if the last file is contained in the first,
then the first is used for error reporting. This requires that these two
overlap to make it easy to find the corresponding spots. Enables going
from

```
within split at mlir/test/IR/invalid.mlir::10 offset :6:9: error: reference to an undefined block
```

to

```
mlir/test/IR/invalid.mlir:15:9: error: reference to an undefined block
```

This does change the split to not produce always null terminated buffers
and tools that need it, need to do so themselves (which is mostly by copying -
this may have little actual impact as previously this was a copy too).

0 files changed, 0 insertions, 0 deletions