[debuginfo] Use byte pos instead of char pos for lineinfo column index

[bjorn3]

Context: https://zulip-archive.rust-lang.org/187780tcompilerwgllvm/51206DWARFdebuglinecolumnandutf8.html

This is the same both gcc and clang are using. It is also a bit faster as there is no need to convert the BytePos from the Span to a CharPos.

[bjorn3]

@rustbot modify labels: +A-debuginfo

[bjorn3]

rust/src/librustc_codegen_llvm/debuginfo/source_loc.rs

Line 27 in f0d4b57

let loc = span_start(bx.cx(), span);

The span_start is responsible for creating a CharPos.