[Static Analyzer] Nullability checker optimization.

[xazax.hun]

Summary

Right now the nullability checker only tracks symbolic regions. For this reason if there are no dead symbols it is safe to skip the precondition checking. I also changed to cleanup code to work with symbols directly instead of memory regions.

lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp

void NullabilityChecker::reportBugIfPreconditionHolds(
   }
 
   reportBug(Error, N, Region, C.getBugReporter(), ValueExpr);
 }
 
 /// Cleaning up the program state.
 void NullabilityChecker::checkDeadSymbols(SymbolReaper &SR,
                                           CheckerContext &C) const {
+  if (!SR.hasDeadSymbols())
+    return;
+
   ProgramStateRef State = C.getState();
   NullabilityMapTy Nullabilities = State->get<NullabilityMap>();
   for (NullabilityMapTy::iterator I = Nullabilities.begin(),
                                   E = Nullabilities.end();
        I != E; ++I) {
-    if (!SR.isLiveRegion(I->first)) {
+    const auto *Region = I->first->getAs<SymbolicRegion>();
+    assert(Region && "Non-symbolic region is tracked.");
+    if (SR.isDead(Region->getSymbol())) {
       State = State->remove<NullabilityMap>(I->first);
     }
   }
   // When one of the nonnull arguments are constrained to be null, nullability
   // preconditions are violated. It is not enough to check this only when we
   // actually report an error, because at that time interesting symbols might be
   // reaped.
   if (checkPreconditionViolation(State, C.getPredecessor(), C))