c++20 compatibility

[phlax]

We reecently updated the Envoy build to make use of c++20

We are still unable to make use of most c++20 features due to the compiler version and issues related to Android ndk for the mobile build

This is causing a lot of clang-tidy failures as its trying to suggest using features that we are not able to

To resolve this we need to

• Update ndk -> r27 when its released (scheduled march 2024)
• Switch mobile -> c++20 (Get Envoy Mobile ready for C++20 #33096)
• Update LLVM - there is a PR to update -> 16.0.0 which should resolve most issues ([WIP] deps: Bump llvm -> 16.0.0 #32712)

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[JuniorHsu]

I'd like to report one issue for C++20
@PeterL328 revealed a weird issue in arm c++20 with some opt build for std::vector<uint8_t>::insert

envoy/test/extensions/filters/listener/tls_inspector/tls_utility.cc

Line 175 in a305c74

clienthello.insert(std::end(clienthello), std::begin(extensions), std::end(extensions));

both clienthello and extensions are std::vector<uint8_t>
the data is inserted in the wrong place, which suggested the c++20 optimizer move data wrongly

visualized to present wrong behavior

clienthello ----client_hello_data----
extensions  aaaaaaaaabbbbbbbbbbcccccccc
post insert ----client_hello_data----bbbbbbnnnnnncccccc

where nnnnnn is noise

the issue is gone if we do
(a) replace insert with for loop + push_back, or
(b) bump clang version from 15 to 18

FWIW, current clang version recommendation is 14

envoy/bazel/README.md

Line 101 in 1095beb

On Linux, we recommend using the prebuilt Clang+LLVM package from [LLVM official site](http://releases.llvm.org/download.html) for Clang 14.