Since IR files are all compiled into separate independent object files in ThinLTO mode,
the prevailing linkonce symbols must be emitted in its object file even if it is no longer
referenced there, e.g. if no references remain in the module after inlining, since it may
be referenced by another ThinLTO compiled object file. This is done by changing
LDPR_PREVAILING_DEF_IRONLY* symbols to LDPR_PREVAILING_DEF, which
converts the prevailing linkonce to weak. We also don't need the other prevailing
IRONLY handling for internalization, which is not currently performed for ThinLTO.
Test case included.