LLVM: Factor out accumulation of LLVM statements and variables

The LLVM code generator currently has a rather large amount of
boilerplate devoted to piping around and building up various AST
elements. This is rather unfortunate for a language which prides itself
on ease of abstraction and detracts from readability.

Here I continue a refactoring that I originally suggested in D991, using
`WriterT` to factor out this pattern. `WriterT` is in general a bit
problematic from an evaluation perspective, but the expressions here are
small enough that it should be a problem in practice.

Test Plan: Validate

Reviewers: austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1286

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