[analyzer] Left shifting a negative value is undefined

[xazax.hun]

Summary

The analyzer did not return an UndefVal in case a negative value was left shifted.
I also added altered the UndefResultChecker to emit a clear warning in this case.

lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp

if (Ex) {
           OS << "a value that is";
         else if (I->isUnsigned())
           OS << '\'' << I->getZExtValue() << "\', which is";
         else
           OS << '\'' << I->getSExtValue() << "\', which is";
 
         OS << " greater or equal to the width of type '"
            << B->getLHS()->getType().getAsString() << "'.";
+      } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
+                 C.isNegative(B->getLHS())) {
+        OS << "The result of the left shift is undefined because the left "
+              "operand is negative";
       } else {
         OS << "The result of the '"
            << BinaryOperator::getOpcodeStr(B->getOpcode())
            << "' expression is undefined";
       }
     }
     auto report = llvm::make_unique<BugReport>(*BT, OS.str(), N);
     if (Ex) {

lib/StaticAnalyzer/Core/BasicValueFactory.cpp

case BO_Sub:
       return &getValue( V1 - V2 );
 
     case BO_Shl: {
 
       // FIXME: This logic should probably go higher up, where we can
       // test these conditions symbolically.
 
       // FIXME: Expand these checks to include all undefined behavior.
+      if (V1.isSigned() && V1.isNegative())
+        return nullptr;
 
       if (V2.isSigned() && V2.isNegative())
         return nullptr;
 
       uint64_t Amt = V2.getZExtValue();
 
       if (Amt >= V1.getBitWidth())
         return nullptr;

test/Analysis/bitwise-ops.c

 }
 
 int testNegativeShift(int a) {
   if (a == -5) {
     return 1 << a; // expected-warning{{The result of the left shift is undefined because the right operand is negative}}
   }
   return 0;
 }
+
+int testNegativeLeftShift(int a) {
+  if (a == -3) {
+    return a << 1; // expected-warning{{The result of the left shift is undefined because the left operand is negative}}
+  }
+  return 0;
+}