__sancov_pcs parallels the other metadata section(s). While some optimizers
(e.g. GlobalDCE) respect linker semantics for comdat and retain or discard the
sections as a unit, some (e.g. GlobalOpt/ConstantMerge) do not. So we have to
conservatively retain all unconditionally in the compiler.
When a comdat is used, the COFF/ELF linkers' GC semantics ensure the
associated parallel array elements are retained or discarded together,
so llvm.compiler.used is sufficient.
Otherwise (MachO (see rL311955/rL311959), COFF special case where comdat is not
used), we have to use llvm.used to conservatively make all sections retain by
the linker. This will fix the Windows problem once internal linkage GlobalObject's in llvm.used are retained via /INCLUDE:.