C identifier name input sections such as __llvm_prf_* are GC roots so
they cannot be discarded. Putting these into a section group disables
the C identifier name semantics.
The ELF section group semantics ensures that group members are retained
or discarded as a unit. When a function symbol is discarded, this allows
allows linker to discard counters, data and values associated with that
function symbol as well.
Note that `noduplicates` COMDAT is lowered to zero-flag section group in
ELF. We only set this for functions that aren't already in a COMDAT and
for those that don't have available_externally linkage since we already
use regular COMDAT groups for those.