Some parts of the test had been extracted into separate files previously.
This patch continues the trend and extracts few more large blocks.
This reduces wall time for the test from a single 14s-long test into a set of
smaller tests that can be run in parallel.
Before/after state of the check-clang tests are here:
https://gist.github.com/Artem-B/d0b05c2e98a49158c02de23f7f4f0279
Differential Revision: https://reviews.llvm.org/D85798
Artem Belevich