Fix another LoopConvert fail.

[angelgarcia]

Summary

Prevent LoopConvert from taking as alias anything that comes from a random member function call.

[alexfh]

LG

clang-tidy/modernize/LoopConvertUtils.cpp

if (OpCall->getOperator() == OO_Star)
       return isDereferenceOfOpCall(OpCall, IndexVar);
     if (OpCall->getOperator() == OO_Subscript) {
       assert(OpCall->getNumArgs() == 2);
       return isIndexInSubscriptExpr(OpCall->getArg(1), IndexVar);
     }
     break;
   }
 
-  case Stmt::CXXMemberCallExprClass:
-    return true;
+  case Stmt::CXXMemberCallExprClass: {
+    const auto *MemCall = cast<CXXMemberCallExpr>(Init);
+    if (MemCall->getMethodDecl()->getName() == "at") {
+      assert(MemCall->getNumArgs() == 1);
+      return isIndexInSubscriptExpr(MemCall->getArg(0), IndexVar);
+    }
+    return false;
+  }
 
   default:
     break;
   }
   return false;
 }
 
 /// \brief Determines whether the bound of a for loop condition expression is

test/clang-tidy/modernize-loop-convert-extra.cpp

void refs_and_vals() {
   // CHECK-FIXES-NOT: MutableVal &{{[a-z_]+}} =
   // CHECK-FIXES: {}
   // CHECK-FIXES-NEXT: alias.x = 0;
 
   dependent<int> dep, other;
   for (dependent<int>::iterator it = dep.begin(), e = dep.end(); it != e; ++it) {
     printf("%d\n", *it);
     const int& idx = other[0];
+    unsigned othersize = other.size();
   }
-  // CHECK-MESSAGES: :[[@LINE-4]]:3: warning: use range-based for loop instead
+  // CHECK-MESSAGES: :[[@LINE-5]]:3: warning: use range-based for loop instead
   // CHECK-FIXES: for (auto & elem : dep)
   // CHECK-FIXES-NEXT: printf("%d\n", elem);
   // CHECK-FIXES-NEXT: const int& idx = other[0];
+  // CHECK-FIXES-NEXT: unsigned othersize = other.size();
+
+  for (int i = 0, e = dep.size(); i != e; ++i) {
+    int idx = other.at(i);
+  }
 }
 
 } // namespace NamingAlias
 
 namespace NamingConlict {
 
 #define MAX(a, b) (a > b) ? a : b
 #define DEF 5