Change dyn_cast<CXXMethodDecl>(FD) to isa<CXXMethodDecl>(FD) [NFC]

The result of the dyn_cast wasn't used to we can just check isa.

llvm-svn: 340531

clang/lib/StaticAnalyzer/Core/RetainSummaryManager.cpp

@@ -326,7 +326,7 @@ RetainSummaryManager::generateSummary(const FunctionDecl *FD,
     }
   }
 
-  if (dyn_cast<CXXMethodDecl>(FD)) {
+  if (isa<CXXMethodDecl>(FD)) {
 
     // Stop tracking arguments passed to C++ methods, as those might be
     // wrapping smart pointers.