file_name -> filename (#3791)

I wanted to choose one or the other. I think some code has been using
each from early on. We're predominately using `filename`, so
consolidating on that. This conveniently matches the [Google dev doc
style guide](https://developers.google.com/style/word-list#filename)
(which we use for docs) which says "filename: Not file name".

In toolchain:

```
╚╡git grep file_name . | wc -l
35
╚╡git grep filename . | wc -l
489
```

toolchain/lex/tokenized_buffer.cpp

@@ -384,7 +384,7 @@ auto TokenizedBuffer::SourceBufferLocationTranslator::GetLocation(
     }
   }
 
-  return {.file_name = buffer_->source_->filename(),
+  return {.filename = buffer_->source_->filename(),
           .line = line,
           .line_number = line_number + 1,
           .column_number = column_number + 1};