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

[Jonathan Dowland]

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

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Commit messages:
 - Adjustments for pre-C++-11
 - 8298730: Refactor subsystem_file_line_contents and add docs and tests

Changes: https://git.openjdk.org/jdk11u-dev/pull/3149/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298730
  Stats: 301 lines in 7 files changed: 242 ins; 22 del; 37 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