IR: Create the Metadata class

This will become the root of a new class hierarchy separate from
`Value`.  As a first step, stick it between `Value` and `MDNode`.

This is part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221886 91177308-0d34-0410-b5e6-96231b3b80d8