IR: Remove MDMapTy and MDPairTy typedefs, NFC

Remove some typedefs in preparation for factoring out attachment logic
from `Instruction`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235764 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed