Add feature tests for C++20 constexpr and consteval This restores support for building pw_rpc with xtensa-esp32-elf-gcc (crosstool-NG esp-2020r3) 8.4.0. No-Docs-Update-Reason: bug fix Change-Id: Ia8af09844855f130e866c2c42f18230da7962118 Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48701 Commit-Queue: Michael Spang <spang@google.com> Reviewed-by: Wyatt Hepler <hepler@google.com>
See our website: http://pigweed.dev