This means we can actually point at a column in the console without it
getting misaligned by multi-byte UTF-8 sequences and Unicode combining
characters.
When using the parser to do static analysis and editor integrations, it's
still useful to be able to get the incomplete AST resulting from a parse
error.
Callers that intend to use the returned value to take real actions (as
opposed to just analysis) must check diags.HasError() to determine if
the returned file can be considered valid.
Currently lacking correct support for unicode text segmentation into
grapheme clusters, so it miscounts "Column" in positions. This will be
addressed later.
