IR: Rewrite uniquing and creation of MDString

Stop using `Value::getName()` to get the string behind an `MDString`.
Switch to `StringMapEntry<MDString>` so that we can find the string by
its coallocation.

This is part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221960 91177308-0d34-0410-b5e6-96231b3b80d8