Revert "C++/ObjC++: switch to gnu++17 as the default standard"

This reverts commit e321c8dd.

This causes many failures in llvm-test-suite, for example:

    /home/npopov/repos/llvm-test-suite/build-O3/tools/timeit --summary MultiSource/Applications/lambda-0.1.3/CMakeFiles/lambda.dir/token_stream.cc.o.time /home/npopov/repos/llvm-project/build/bin/clang++ -DNDEBUG -I/home/npopov/repos/llvm-test-suite/MultiSource/Applications/lambda-0.1.3 -O3   -w -Werror=date-time -MD -MT MultiSource/Applications/lambda-0.1.3/CMakeFiles/lambda.dir/token_stream.cc.o -MF MultiSource/Applications/lambda-0.1.3/CMakeFiles/lambda.dir/token_stream.cc.o.d -o MultiSource/Applications/lambda-0.1.3/CMakeFiles/lambda.dir/token_stream.cc.o -c /home/npopov/repos/llvm-test-suite/MultiSource/Applications/lambda-0.1.3/token_stream.cc
    /home/npopov/repos/llvm-test-suite/MultiSource/Applications/lambda-0.1.3/token_stream.cc:192:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
            register char chr;
            ^~~~~~~~~