[Alignment] Add a None() member function

Summary:
This will allow writing `if(A != llvm::Align::None())` which is clearer than `if(A > llvm::Align(1))`

This is patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790

Reviewers: courbet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67697

llvm-svn: 372207