[Alignment] Get DataLayout::StackAlignment as Align

Summary:
Internally it is needed to know if StackAlignment is set but we can
expose it as llvm::Align.

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: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67852

llvm-svn: 372585