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

The subdirectory has a lit.local.cfg that marks the tests unsupported
if llvm was built without Asserts.  There will be a patch in LLVM
that disables statistics gathering when built without Asserts so
that full Release builds can be faster.  Statistics can also
be enabled by building with -DLLVM_ENABLE_STATS.