Partly revert "Move clang tests that depend on llvm/ADT/Statistic.h to a subdir".

This reverts commit 176730, and uses "REQUIRES: asserts" instead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176815 91177308-0d34-0410-b5e6-96231b3b80d8