StaticAnalyzer: fix compiler warning. NFC

My compiler (clang-3.8) complains that the RCC variable is unused.
That's not really true, as it's checked by the if-declaration, but it's
also kinda true, because we don't need to declaration if we only check
it in the if statement.

In reality, all this means that the dyn_cast<> can be replaced by isa<>,
so that's what I do here.

llvm-svn: 327491

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

@@ -464,7 +464,7 @@ ProgramStateRef ExprEngine::addAllNecessaryTemporaryInfo(
 
     // If the temporary is being returned from the function, it will be
     // destroyed or lifetime-extended in the caller stack frame.
-    if (const auto *RCC = dyn_cast<ReturnedValueConstructionContext>(CC)) {
+    if (isa<ReturnedValueConstructionContext>(CC)) {
       const StackFrameContext *SFC = LC->getCurrentStackFrame();
       assert(SFC);
       if (SFC->getParent()) {