4276: Don't count start of non-ASCII characters as being inside of them r=matklad a=lnicola
I'm still not sure that `utf16_to_utf8_col` is correct for code points from Supplementary Planes. These have two UTF-16 code units, and I feel we're not going to count them correctly.
Fixes the crash in https://github.com/rust-analyzer/rust-analyzer/issues/4263#issuecomment-622988258.
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>