AsmWriter: Split out code for printing Metadata attachments, NFC

Refactor the code for printing `Instruction` metadata attachments so it
can be reused for `Function`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235772 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed