[Alignment][NFC] Remove AllocaInst::setAlignment(unsigned)

[Alignment][NFC] Remove AllocaInst::setAlignment(unsigned)

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

Subscribers: jholewinski, arsenm, jvesely, nhaehnle, eraman, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D68141

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373207 91177308-0d34-0410-b5e6-96231b3b80d8