Clang: C++20 flag for Clang 11.0+, record full C++17 support for Clang 6.0+
-
-std=c++20
supersedes-std=c++2a
in Clang 11.0 (commit). Use that for Clang 11.0+. - Clang 6.0+ fully support C++17 language features per the cppreference.com C++17 compiler support table.
Backport: release