disable all CURRENT compilation warnings in 21.2

As agreed in the analysis release meeting the specific goal here is just to disable all the warnings that have existed in 21.2 for a long time and that lead to occasional confusion during the merge reviews. In a few cases I tried to address the actual source of the warning, but for the most part I just picked the most expedient way to shut it off, instead of trying to determine whether they point to an actual problem or not, and then fixing them accordingly.

Maybe as a special instruction for the shifter: The goal is really just to get the compilation warning-free, it is not meant to be particularly pretty, as it mostly affects only files that have not been touched in a long time and will likely not see any more changes in 21.2. As such it does contain things we wouldn't normally allow into the repository, like pragmas to disable warnings and commented out code.