Change the default leap C++ compilation flag to -std=c++20

[greg7mdp]

Prerequisites:

• Proposal for Leap build requirements to support C++20 #1084
• Update Leap and submodules, so that they build with -std=c++20.  #922
• c++20 support - Add fixed header file from llvm-11 distrib and use in build. #1121 via (C++20 support - Build with patched header file from llvm-11 distrib (STLExtras.h) #1116 OR C++20 support - continue to use c++17 on files that use LLVM #1192)
• C++20 support - add boost 1.82 as a submodule #1142
• Start new version of nodeos (compiled with C++20) from old version (generated by nodeos compiled with C++17) state, check integrity hash, ensure it can still sync a few blocks afterward. (Run this test with pinned clang-11 and also gcc-10, or whatever minimum versions we decide in Proposal for Leap build requirements to support C++20 #1084.)

llvm issue background

This is a known build issue with the LLVM headers (gcc-12, -std=c++20), see this.

Manually changing /usr/include/llvm-11/llvm/ADT/STLExtras.h to remove the unneeded <R> allows it to build fine.

Issue is fixed in LLVM-13

Possible way forward

Investigate what part of leap depends on those llvm headers, and possibly keep building these with -std=c++17.

[arhag]

We also want to run a replay (even before chicken test is ready) soon after making the C++20 switch.

[greg7mdp]

Issue with g++-12 / std=c++20

Investigate what part of leap depends on those llvm headers, and possibly keep building these with -std=c++17.

I tried this, setting set_property(TARGET eosio_chain PROPERTY CXX_STANDARD 17) in libraries/chain/CMakeLists.txt.
While this does fix the compilation issue for the cpp files, it creates new link issues such as:

usr/bin/ld: libraries/chain/libeosio_chain.a(crypto.cpp.o): in function `eosio::chain::webassembly::interface::alt_bn128_mul(eosio::vm::span<char const, 18446744073709551615ul>, eosio::vm::span<char const, 18446744073709551615ul>, eosio::vm::span<char, 18446744073709551615ul>) const':
/home/greg/github/enf/leap/libraries/chain/webassembly/crypto.cpp:130: undefined reference to `bn256::g1_scalar_mul(nonstd::span_lite::span<unsigned char const, 64ul>, nonstd::span_lite::span<unsigned char const, 32ul>, nonstd::span_lite::span<unsigned char, 64ul>)'
/usr/bin/ld: libraries/chain/libeosio_chain.a(crypto.cpp.o): in function `eosio::chain::webassembly::interface::alt_bn128_pair(eosio::vm::span<char const, 18446744073709551615ul>) const':
/home/greg/github/enf/leap/libraries/chain/webassembly/crypto.cpp:139: undefined reference to `bn256::pairing_check(nonstd::span_lite::span<unsigned char const, 18446744073709551615ul>, std::function<void ()>)'
collect2: error: ld returned 1 exit status

I tried building with llvm-13 bit the headers have changed:

/home/greg/github/enf/leap/libraries/chain/webassembly/runtimes/eos-vm-oc/LLVMJIT.cpp:23:10: fatal error: llvm/ExecutionEngine/Orc/LambdaResolver.h: No such file or directory
   23 | #include "llvm/ExecutionEngine/Orc/LambdaResolver.h"

Also, even when patching the headers by hand, some tests fail in release mode with the g++-12 / std=c++20

The following tests FAILED:
	 94 - partitioned_block_log_unit_test_eos-vm-oc (SEGFAULT)
	 95 - partitioned_block_log_unit_test_eos-vm (SEGFAULT)
	 96 - partitioned_block_log_unit_test_eos-vm-jit (SEGFAULT)

However g++-10 / std=c++20 is fine - as is clang-16.

• leap builds fine (and without a single warning). It is a thing of beauty.
• tests all pass (release / debug)

Proposal

1. standardize on g++-10 / clang-16 for compiler versions
2. flip the switch to --std=c++20

[bhazzard]

Blocked awaiting completion of Prerequisites (in description)