[Diag] Add getter shouldAlwaysPrint. NFC

For the new hotness attribute, the API will take the pass rather than
the pass name so we can no longer play the trick of AlwaysPrint being a
special pass name. This adds a getter to help the transition.

There is also a corresponding llvm patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274101 91177308-0d34-0410-b5e6-96231b3b80d8