[Alignment] Introduce llvm::Align to MCSection

Summary:
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, JDevlieghere

Subscribers: arsenm, sdardis, jvesely, nhaehnle, sbc100, hiraditya, aheejin, jrtc27, atanasyan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67486

llvm-svn: 371831