[Linker] Handle comdat nodeduplicate

For a variable in a comdat nodeduplicate, its initializer may be significant.
E.g. its content may be implicitly referenced by another comdat member (or
required to parallel to another comdat member by the runtime when explicit
section is used). We can clone it into an unnamed private linkage variable to
preserve its content.

This partially fixes PR51394 (Sony's proprietary linker using LTO): no error
will be reported. This is partial because we do not guarantee the global
variable order if the runtime has parallel section requirement.

---

There is a similar issue for regular LTO, but unrelated to PR51394:

with lib/LTO (using either ld.lld or LLVMgold.so), linking two modules
with a weak function of the same name, can leave one weak profc and two
private profd, due to lib/LTO's current deficiency that it mixes the two
concepts together: comdat selection and symbol resolution. If the issue
is considered important, we should suppress private profd for the weak+
regular LTO case.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D108879