[Utils] Add a switch controlling prefix warnings in UpdateTestChecks

The switch controls both unused prefix warnings, and warnings about
functions which differ under different runs for a prefix, and, thus, end
up not having asserts for that prefix.

(If the latter case spans to all functions, then the former case kicks
in)

The switch is on by default, and can be disabled.

Differential Revision: https://reviews.llvm.org/D95829