RFR: 8283325: US_ASCII decoder relies on String.decodeASCII being exhaustive [v2]

[Claes Redestad]

> Adjust `String.decodeASCII` to deal with the possibility that `StringCoding.countPositives` can return a value less than the exact number of leading positive bytes. This is the likely cause of a number of intermittent but somewhat common errors on aarch64 tests in our CI.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Add comment, remove redundant Arrays.fill

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7855/files
  - new: https://git.openjdk.java.net/jdk/pull/7855/files/aa0470cf..ef9fbaa7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7855&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7855&range=00-01

  Stats: 6 lines in 1 file changed: 5 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7855.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7855/head:pull/7855

PR: https://git.openjdk.java.net/jdk/pull/7855