RFR: 8256828: ostream::print_cr() truncates buffer in copy-through case [v4]

[David Holmes]

> Clarifies the behaviour of this function in regards to truncation when adding a CR. Ensures a truncation warning is always issued.
> 
> Adds unit testing for the specialized cases.
> 
> See JBS for discussion of other suggestions.
> 
> Testing: - tiers 1-4
> 
> Thanks

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Simplified the test code - thanks @tstuefe!
  Rewrote the comment block describing do_vsnprintf.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19512/files
  - new: https://git.openjdk.org/jdk/pull/19512/files/eeee7f57..1aef14ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19512&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19512&range=02-03

  Stats: 43 lines in 3 files changed: 5 ins; 1 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/19512.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19512/head:pull/19512

PR: https://git.openjdk.org/jdk/pull/19512