ModuleUtils: Stop using comdat members to generate unique module ids.

ModuleUtils: Stop using comdat members to generate unique module ids.

It is possible for two modules to define the same set of external
symbols without causing a duplicate symbol error at link time,
as long as each of the symbols is a comdat member. So we cannot
use them as part of a unique id for the module.

Differential Revision: https://reviews.llvm.org/D38602

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315026 91177308-0d34-0410-b5e6-96231b3b80d8