Optimize pretty printing performance

[bnbarham]

#883 fixed outputting incorrect line numbers, but introduced a performance regression. #901 improved this back to around the original, but had to be reverted as it introduced a an issue due to counting codepoints rather than characters.

Introduce a similar optimization again, but only for the first portion of the string (prior to the last newline).

Fixes #894 again.

[bnbarham]

CC @macshome

[bnbarham]

Some instruction counts (for formatting swift-syntax 509.0.0):

• Original before PrettyPrinter reports wrong line LineNumbersTests #883 was 67679344706
• After PrettyPrinter reports wrong line LineNumbersTests #883 was 71966461520
• And after this it's now 69488311542

Still worse than it was, but we're doing a fair bit more work so that seems inevitable.

[rintaro]

I would do:

    if let lastNewlineIndex = text.utf8.lastIndex(of: UInt8(ascii: "\n")) {
      let lastLineStart = text.utf8.index(after: lastNewlineIndex)
      lineNumber += text[..<lastLineStart].utf8.count(where: {$0 == UInt8(ascii: "\n")})
      column = text[lastLineStart...].count
    } else {
      column += text.count
    }

[bnbarham]

Ah, thanks @rintaro. We're now down to 68032409793 👍