[NFC] Disallow unused prefixes under llvm/test

This patch sets the default for llvm tests, with the exception of tests
under Reduce, because quite a few of them use 'FileCheck' as parameter
to a tool, and including a flag as that parameter would complicate
matters.

The rest of the patch undo-es the lit.local.cfg changes we progressively
introduced as temporary measure to avoid regressions under various
directories.

Differential Revision: https://reviews.llvm.org/D95111