[jdk11u-dev] RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests [v2]

[Jonathan Dowland]

> Not clean, adjustments necessary to adapt for pre-C++-11, and a small change to account for lack of JEP 371.

Jonathan Dowland has updated the pull request incrementally with one additional commit since the last revision:

  different approach for initializing _is_frozen
  
  G++ does not like a trailing ',' in the member initialiser list,
  which occurs for non-debug builds when expanding DEBUG_ONLY.
  
  Move the initialisation of _is_fixed into the constructor
  definitions.
  
  Signed-off-by: Jonathan Dowland <jdowland at redhat.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk11u-dev/pull/3149/files
  - new: https://git.openjdk.org/jdk11u-dev/pull/3149/files/d912190a..39940791

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=00-01

  Stats: 6 lines in 1 file changed: 2 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3149.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3149/head:pull/3149

PR: https://git.openjdk.org/jdk11u-dev/pull/3149