Rollup merge of #136460 - real-eren:simplify-rustc_span-analyze, r=Noratrieb

Simplify `rustc_span` `analyze_source_file`

Simplifies the logic to what the code *actually* does, which is to just record newlines and multibyte characters. Checking for other ASCII control characters is unnecessary because the generic fallback doesn't do anything for those cases.
Also uses a simpler (and more efficient) means of iterating the set bits of the mask.

Author: workingjubilee