IR: Allow multiple global metadata attachments with the same type.

IR: Allow multiple global metadata attachments with the same type.

This will be necessary to allow the global merge pass to attach
multiple debug info metadata nodes to global variables once we reverse
the edge from DIGlobalVariable to GlobalVariable.

Differential Revision: http://reviews.llvm.org/D20414

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271358 91177308-0d34-0410-b5e6-96231b3b80d8