[PECOFF] Only COMDAT symbols are allowed to be dead-stripped.

We should dead-strip atoms only if they are created for COMDAT symbols. If we
remove non-COMDAT atoms from a binary, it will no longer be guaranteed that
the binary will work correctly.

In COFF, you can manipulate the order of section contents in the resulting
binary by section name. For example, if you have four sections
.data$unique_prefix_{a,b,c,d}, it's guaranteed that the contents of A, B, C,
and D will be consecutive in the resulting .data section in that order.
Thus, you can access B's and C's contents by incrementing a pointer pointing
to A until it reached to D. That's why we cannot dead-strip B or C even if
no one is directly referencing to them.

Some object files in the standard library actually use that technique.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193017 91177308-0d34-0410-b5e6-96231b3b80d8