[lld] Discard SHT_LLVM_LTO sections in relocatable links (#92825)

So long as ld -r links using bitcode always result in an ELF object, and
not a merged bitcode object, the output form a relocatable link using
FatLTO objects should not have a .llvm.lto section. Prior to this, using
the object code sections would cause the bitcode section in the output
of a relocatable link to be corrupted, by concatenating all the
.llvm.lto
sections together.

This patch discards SHT_LLVM_LTO sections when not using
--fat-lto-objects, so that the relocatable ELF output won't contain
inalid bitcode.