Handle a peculiar comdat case: Creating a section with an undefined

Handle a peculiar comdat case: Creating a section with an undefined
signature symbol causes a local symbol to be created unless there is
some other use of the symbol.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119026 91177308-0d34-0410-b5e6-96231b3b80d8