RFR: 8314488: Compile the JDK as C++17 [v6]

[Julian Waters]

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

Julian Waters has updated the pull request incrementally with one additional commit since the last revision:

  Require clang 13 in toolchain.m4

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14988/files
  - new: https://git.openjdk.org/jdk/pull/14988/files/4f196292..50ffeeea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=04-05

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