Disable clang-format's MemoizationTest as it becomes prohibitive with EXPENSIVE_CHECKS

EXPENSIVE_CHECKS enables libstdc++'s library consistency checks, which
includes checking the container passed to std::priority_queue for its
well-formedness. This makes the clang-format memoization too expensive,
so disable it.

(it's a necessary feature of libstdc++'s consistency checks that it
ruins the required scalability of C++ standard library features - so
these workarounds are to be expected if a test ever tries to test
scalability in some way, like this test does)

llvm-svn: 312268