[lld-macho] Folded symbols should have size zero in linker map

This matches ld64's behavior.

I also extended the icf-stabs.s test to demonstrate that even though
folded symbols have size zero, we cannot use the size-zero property in
lieu of `wasIdenticalCodeFolded`, because size zero symbols should still
get STABS entries.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D136001