[mlir] Support opaque types in LLVM IR -> MLIR translation

LLVM IR is moving towards adoption of opaque pointer types. These require extra
information to be passed when constructing some operations, in particular GEP
and Alloca. Adapt the builders of said operations and modify the translation
code to handle both opaque and non-opaque pointers.

This incidentally adds the translation for Alloca alignment and fixes the translation
of struct-related GEP indices that must be constant.

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D123792