I think we would be better off with tests explicitly specifying the
language mode. Right now Lang_C means C99, but reads as "any C version",
or as "unspecified C version".
I also changed '-std=c++98' to '-std=c++03' because they are aliases (so
there is no difference in practice), because Clang implements C++03
rules in practice, and because 03 makes a nice sortable progression
between 03, 11, 14, 17, 20.