[clang] Include clang config.h in LangStandards.cpp

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.

Reviewed By: hokein, dexonsmith

Differential Revision: https://reviews.llvm.org/D124974