Make BUILD.gn targets into templates. This allows us to "globally" add and remove certain configs as long as we use the new templates. This simplifies the logic of adding configs for stuff like extra warnings and default include dirs. As well it simplifies removing certain common unwanted configs. Generally simplifies the logic in BUILD.gn. Will allow for easily suppressing the clang-plugins config instead of using a global setting in .gn. Then we can enable the additional warnings config-by-config. Also fixes some warnings that turned up after we enabled the extra warnings config in our tests. Also moves the dEQP tests main to be consistent with the other test main files. Bug: angleproject:3069 Change-Id: I5a8166cd0f5a7926822c171fcaf473fc86b3ffc1 Reviewed-on: https://chromium-review.googlesource.com/c/1409871 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>