Reland "Actually build libc++ and libc++abi with -std=c++20"

This is a reland of d95d20a3f7cc630e324e706c0a2bd405792283c4
https://chromium-review.googlesource.com/c/chromium/src/+/3497608
fixed the Mac/debug test failure.

Original change's description:
> Actually build libc++ and libc++abi with -std=c++20
>
> build/config/compiler/BUILD.gn adds the flag to cflags_cc, so this
> config here must do that too. Else the -std=c++20 will show up
> on the commandline _before_ the -std=c++17 added by
> build/config/compiler:compiler, rendering it ineffective.
>
> Bug: 1298070
> Change-Id: I145b04aeef1003620594a0be8f74bb7731797069
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3469402
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Arthur Eubanks <aeubanks@google.com>
> Commit-Queue: Arthur Eubanks <aeubanks@google.com>
> Cr-Commit-Position: refs/heads/main@{#972250}

Bug: 1298070
Change-Id: I03e4c4978aceba73868704f6e019702e7772723a
Cq-Include-Trybots: luci.chromium.try:gpu-try-mac-intel-dbg
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3485017
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#976946}
NOKEYCHECK=True
GitOrigin-RevId: d478debaaefae8c15ab83e666f4823de164162b8
1 file changed