RFR: 8302863: Speed up String::encodeASCII using countPositives

[Claes Redestad]

When encoding Strings to US-ASCII we can speed up the happy path significantly by using `StringCoding.countPositives` as a speculative check for whether there are any chars that needs to be replaced by `'?'`. Once a non-ASCII char is encountered we fall back to the slow loop and replace as needed.

An alternative could be unrolling or using a byte array VarHandle, as show-cased by Brett Okken here: https://mail.openjdk.org/pipermail/core-libs-dev/2023-February/100573.html Having to replace chars with `?` is essentially an encoding error so it might be safe to assume this case is exceptional in practice.

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Commit messages:
 - Cleanup, clone
 - Merge branch 'master' into encodeASCII
 - Improve encodeASCII by leveraging countPositives

Changes: https://git.openjdk.org/jdk/pull/12640/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12640&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302863
  Stats: 24 lines in 1 file changed: 11 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12640.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12640/head:pull/12640

PR: https://git.openjdk.org/jdk/pull/12640