Fix a crash/mislex when more than one frontmatter closing possibility is considered
When the less fortunate recovery path for frontmatters are taken, if the lexer considers more than one possible frontmatter closing possibility, the current index is entirely mis-tracked and can result in bump_bytes landing in the middle of a multichar unicode character.
This fixes it by tracking the actual base index and updating it as it considers additional closing possibilities.
fixesrust-lang/rust#146847
f984966325