Since the only missing bit was the size, I just replaced the Elf_Sym with the size.
We could go with DefinedCommon, DefinedCommonElf<ELFT> and DefinedCommonBitcode, but that seems a bit much.
Sending email since I forgot to add llvm-commits initially.
LGTM with a nit
I dont think we used const for auto anywhere in lld before.