[Alignment][NFC] Migrate CallingConv tablegen code

Summary:
We first migrate the generated code, more patches to come.

This 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

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81196