[jdk11u-dev] RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests [v4]

[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:

  remove potentially misleading comment for 11u (thanks Severin)

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Changes:
  - all: https://git.openjdk.org/jdk11u-dev/pull/3149/files
  - new: https://git.openjdk.org/jdk11u-dev/pull/3149/files/8564738f..f126e0b7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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