[CI] Enable -Werror where possible

Summary:
The CI build-werror plan turns some warnings into error according to
what is defined by the cmake ENABLE_WERROR option, and only applies to
the default target built with Clang.
This diffs turns that build plan into a clang-10 build plan instead, and makes
the CI use the -Werror flag over all the regular builds. This will allow for preventing
warnings to get introduced for clang-8, clang-10 and gcc-8, with various flag
configurations.

Test Plan: Run all the "regular" CI build plans.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6315