IR: Disallow function-local metadata attachments

Metadata attachments to instructions cannot be function-local.

This is part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223574 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid angle brackets in comment. [-Wdocumentation-html]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223576 91177308-0d34-0410-b5e6-96231b3b80d8