To implement incremental document syncing, we want to verify that the
ranges provided by the front-end are valid. Currently, positionToOffset
deals with invalid Positions by returning 0 or Code.size(), which are
two valid offsets.
Instead, return an llvm:Expected<size_t> with an error if the position
is invalid. According to the LSP, if the character value exceeds the
number of characters of the given line, it should default back to the
end of the line, so this is what I did. However, I considered a line
number larger than the number of lines in the file to be an error.