THRIFT-2676 Avoid 'i386' name collision in generated Cocoa/objc code

[THRIFT-2676](https://issues.apache.org/jira/browse/THRIFT-2676)

This fixes a bug in the cocoa code generator where the variable
used by a for-loop can conflict with a built-in symbol when the
temporary variable counter is equal to 386. The generated variable
name, 'i386', conflicts with a macro built-in to the compiler.

I can reproduce this bug on Xcode 5 as well as Xcode 6. It appears
to only affect iOS projects, not OS X projects.

My fix simply prefixes the generated variable with 'idx' instead of 'i'.

This test code demonstrates the problem, regardless of Thrift codegen.

```
int i386 = 42;
printf("foobar %d\n", i386);
```

Which results in the following compiler error:

```
/Users/keith/Desktop/ReservedSymbolTest/ReservedSymbolTest/ViewController.m:22:7: error: expected identifier or '('
  int i386 = 42;
      ^
<built-in>:143:14: note: expanded from here
             ^
1 error generated.
```