[NFC] factor update test function test builder as a class

This allows us to have shared logic over multiple test runs, e.g. do we
have unused prefixes, or which function bodies have conflicting outputs
for a prefix appearing in different RUN lines.

This patch is just wrapping existing functionality, and replacing its uses.
A subsequent patch would then fold the current functionality into the newly
introduced class.

Differential Revision: https://reviews.llvm.org/D93413