[lld] Explicitly ignore comdat groups when parsing LTO object(s)

Any symbols defined in the LTO object are by definition the ones we
want in the final output so we skip the comdat group checking in those
cases.

This change makes the ELF code more explicit about this and means
that wasm and ELF do this in the same way.

Differential Revision: https://reviews.llvm.org/D62884

llvm-svn: 362625