IR: Make MDString::getName() private

Hide the fact that `MDString`'s string is stored in `Value::Name` --
that's going to change soon.  Update the only in-tree client that was
using it instead of `Value::getString()`.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221951 91177308-0d34-0410-b5e6-96231b3b80d8