AsmWriter: Only collect attachment names once per module

Collect metadata names once per `AssemblyWriter` instead of every time
we need to print some attachments.  Just a drive-by; this caught my eye
while I was refactoring the code in r235772.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235774 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed