Fix lock rank violation assersions

There's a race in the emitter, where if we emit a unit (because,
it wasn't in the repo) fail to commit it, and then find the unit in the repo,
we destroy the unit we just emitted, and read the one from the repo instead.

This typically happens at RankFileRepo. A new change to Func::~Func makes
it grab the write lease in DEBUG builds, which is a lock rank violation.

Newly created Funcs never need the write lease, however, so aquire it
lazily.

This was happening running slow tests, because some tests are run multiple
times (potentially in parallel) with different runtime options (in fact, the
test code is duplicated in separate files).

hphp/runtime/vm/jit/translator-x64.cpp

@@ -1328,12 +1328,25 @@ funcPrologSmashGuard(TCA prolog, const Func* func) {
 void
 TranslatorX64::smashPrologueGuards(TCA* prologues, int numPrologues,
                                    const Func* func) {
-#ifdef DEBUG
-  LeaseHolder writer(s_writeLease);
-#endif
+  DEBUG_ONLY std::unique_ptr<LeaseHolder> writer;
   for (int i = 0; i < numPrologues; i++) {
     if (prologues[i] != (TCA)fcallHelperThunk
         && funcPrologHasGuard(prologues[i], func)) {
+      if (debug) {
+        /*
+         * Unit's are sometimes created racily, in which case all
+         * but the first are destroyed immediately. In that case,
+         * the Funcs of the destroyed Units never need their
+         * prologs smashing, and it would be a lock rank violation
+         * to take the write lease here.
+         * In all other cases, Funcs are destroyed via a delayed path
+         * (treadmill) and the rank violation isn't an issue.
+         *
+         * Also note that we only need the write lease because we
+         * mprotect the translation cache in debug builds.
+         */
+        if (!writer) writer.reset(new LeaseHolder(s_writeLease));
+      }
       funcPrologSmashGuard(prologues[i], func);
     }
   }