Replace a dyn_cast with isa where the result was only being used as a boolean. NFC.

llvm-svn: 253442
diff --git a/clang-tools-extra/clang-tidy/modernize/UseDefaultCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDefaultCheck.cpp
index 098f864..68ead15 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDefaultCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDefaultCheck.cpp
@@ -302,7 +302,7 @@
       if (!StartLoc.isValid())
         return;
     }
-  } else if (dyn_cast<CXXDestructorDecl>(SpecialFunctionDecl)) {
+  } else if (isa<CXXDestructorDecl>(SpecialFunctionDecl)) {
     SpecialFunctionName = "destructor";
   } else {
     if (!isCopyAssignmentAndCanBeDefaulted(Result.Context, SpecialFunctionDecl))