Improve input validation checks and documentation of the `java.lang.StringUTF16` intrinsics.
------------- Commit messages: - `with_opaque` of `generate_string_range_check` become redundant, remove it - Sharpen class visibility - Guard `getChars()` - Guard `toBytes()` - Factor bailing logic in `generate_string_range_check` out - Document `getChar`/`putChar` Changes: https://git.openjdk.org/jdk/pull/30139/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=30139&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8378431 Stats: 243 lines in 5 files changed: 100 ins; 52 del; 91 mod Patch: https://git.openjdk.org/jdk/pull/30139.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/30139/head:pull/30139 PR: https://git.openjdk.org/jdk/pull/30139
