RFR: 8301958: Avoid Arrays.copyOfRange overhead in java.lang.String

[Claes Redestad]

This adds a local, specialized `copyBytes` method to `String` that avoids certain redundant range checks and clamping that JIT has issues removing fully.

This has a small but statistically significant effect on `String` microbenchmarks, eg.:

Baseline

Benchmark                                            (size)  Mode  Cnt   Score   Error  Units
StringConstructor.newStringFromArray                      7  avgt   15  16.817 ± 0.369  ns/op
StringConstructor.newStringFromArrayWithCharset           7  avgt   15  16.866 ± 0.449  ns/op
StringConstructor.newStringFromArrayWithCharsetName       7  avgt   15  22.198 ± 0.396  ns/op


Patch:

Benchmark                                            (size)  Mode  Cnt   Score   Error  Units
StringConstructor.newStringFromArray                      7  avgt   15  15.477 ± 0.342  ns/op
StringConstructor.newStringFromArrayWithCharset           7  avgt   15  15.557 ± 0.352  ns/op
StringConstructor.newStringFromArrayWithCharsetName       7  avgt   15  21.272 ± 0.398  ns/op


Care has to be taken to ensure preconditions have been checked when using `checkBytes`. In the case of `String(AbstractStringBuilder)` there's a possible pre-existing issue where the constructor might either throw an exception or truncate the buffer if the builder byte array and length is not in agreement (theoretically possible if you clear/remove and call `trimToSize()` concurrently). Adding an explicit check here seem to be the right thing to do regardless of this RFE.

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Commit messages:
 - Include some callsites in StringLatin1/UTF16, rename to copyBytes to disambiguate from generic utility methods, tune microbenchmark
 - 8301958: Avoid overhead of Arrays.copyOfRange in String

Changes: https://git.openjdk.org/jdk/pull/12453/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301958
  Stats: 36 lines in 4 files changed: 14 ins; 2 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/12453.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12453/head:pull/12453

PR: https://git.openjdk.org/jdk/pull/12453