forked from boostorg/core
clang-8 jobs fail in C++20 mode since it doesn't support std::is_constant_evaluated, which is called from libstdc++-10 that is installed in Focal image. Moving it back to Bionic should work around the issue as it should use an older libstdc++ version. gcc-10 and 11 were moved to Focal just to make this configuration tested. Also, changed 2a to 20 for compilers that support this.