Fix `SourceFile::normalized_byte_pos`
This method was broken by 258ace6, which changed `self.normalized_pos` to use relative offsets however this method continued to compare against an absolute offset.
Also adds a regression test for the issue that this method was originally introduced to fix.
Closesrust-lang/rust#149568
Fixes regression of rust-lang/rust#110885
r? cjgillot (as author of the breaking commit)