AsmWriter: Parameterize the syntactic separator for attachments

Parameterize the separator for attachments, since `Function` metadata
attachments (PR23340) aren't going to use a `,` (comma).  No real
functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235775 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed