RFR: 8314488: Compile the JDK as C++17 [v4]

[Julian Waters]

> Compile the JDK as C++17, enabling the use of all C++17 language features

Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'openjdk:master' into patch-7
 - Compiler versions in toolchain.m4
 - Merge branch 'openjdk:master' into patch-7
 - Merge branch 'openjdk:master' into patch-7
 - Revert vm_version_linux_riscv.cpp
 - vm_version_linux_riscv.cpp
 - allocation.cpp
 - 8310260

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14988/files
  - new: https://git.openjdk.org/jdk/pull/14988/files/477f6b94..f1a644e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=02-03

  Stats: 20857 lines in 728 files changed: 13941 ins; 3659 del; 3257 mod
  Patch: https://git.openjdk.org/jdk/pull/14988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14988/head:pull/14988

PR: https://git.openjdk.org/jdk/pull/14988