[FileCheck] Disallow unused prefixes in llvm/test/Analysis

This is achieved through a substitution of FileCheck in lit.cfg.py,
where we explicitly set -allow-unused-prefixes to false.

We also introduce a %FileCheckWithUnusedPrefixes% substitution that can
be used in those cases where we want to allow unused prefixes, even if
the folder policy is to disallow them.

Differential Revision: https://reviews.llvm.org/D91275