Add CI for the C++20/Clang/libstdc++ combination

PiperOrigin-RevId: 725253209
Change-Id: I52cab63f3c1011716f87dbd0200187414e086ae3

Author: derekmauro

ci/linux_clang-latest_libstdcxx_bazel.sh

@@ -25,7 +25,7 @@ if [[ -z ${ABSEIL_ROOT:-} ]]; then
 fi
 
 if [[ -z ${STD:-} ]]; then
-  STD="c++17"
+  STD="c++17 c++20"
 fi
 
 if [[ -z ${COMPILATION_MODE:-} ]]; then