[clang][Sema][NFC] Make some locals const in getUndefinedButUsed()

Author: tbaederr

clang/lib/Sema/Sema.cpp

@@ -825,7 +825,7 @@ void Sema::getUndefinedButUsed(
       continue;
     }
 
-    if (FunctionDecl *FD = dyn_cast<FunctionDecl>(ND)) {
+    if (const auto *FD = dyn_cast<FunctionDecl>(ND)) {
       if (FD->isDefined())
         continue;
       if (FD->isExternallyVisible() &&
@@ -836,7 +836,7 @@ void Sema::getUndefinedButUsed(
       if (FD->getBuiltinID())
         continue;
     } else {
-      auto *VD = cast<VarDecl>(ND);
+      const auto *VD = cast<VarDecl>(ND);
       if (VD->hasDefinition() != VarDecl::DeclarationOnly)
         continue;
       if (VD->isExternallyVisible() &&