CGCXX: Use cast in getAddrOfCXXStructor()

All callers should be passing `CXXConstructorDecl` or
`CXXDestructorDecl` here, so use `cast<>` instead of `dyn_cast<>` when
setting up the `GlobalDecl`.

llvm-svn: 236651
diff --git a/clang/lib/CodeGen/CGCXX.cpp b/clang/lib/CodeGen/CGCXX.cpp
index c8f2629..29a199d 100644
--- a/clang/lib/CodeGen/CGCXX.cpp
+++ b/clang/lib/CodeGen/CGCXX.cpp
@@ -231,8 +231,7 @@
   if (auto *CD = dyn_cast<CXXConstructorDecl>(MD)) {
     GD = GlobalDecl(CD, toCXXCtorType(Type));
   } else {
-    auto *DD = dyn_cast<CXXDestructorDecl>(MD);
-    GD = GlobalDecl(DD, toCXXDtorType(Type));
+    GD = GlobalDecl(cast<CXXDestructorDecl>(MD), toCXXDtorType(Type));
   }
 
   StringRef Name = getMangledName(GD);