Relax language comparison when matching up C++ forward decls with definitions

when dealing with -gmodules debug info.

This fixes the bot failures on Darwin.

A recent clang change (presumably https://reviews.llvm.org/D104291)
introduced a bug where .pcm files would identify themselves as
DW_LANG_C_plus_plus, but the .o that references them would identify as
DW_LANG_C_plus_plus_14. While that bug needs to be fixed, too, it
shows that the current strict comparison also isn't meaningful.

rdar://79423225