Make sure we're using the same version of clang tools for everything

Summary:
clang-7 is installed by default. We should use the same version of clang and various clang tools.
This patch bumps clang to 8 to match the other tools.

Depends on D5835

Test Plan: Run various clang test suites on CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5836