This is necessary in order to pick up the default C/C++ standard from
the CLANG_DEFAULT_STD_C(XX) defines. This fixes a bug that was
introduced when this default language standard code was moved from
Frontend to Basic, making compilers ignore the configured default
language version override.
Fixes a bug introduced by D121375.
I just noticed that the other comments in this file are C-style. Probably the new ones should be too.