Revert "[clang] Build UsingType for elaborated type specifiers."

This reverts commit e70ca7b3 and the
followup patch "[clang] Fix the location of UsingTypeLoc"
(ebbeb164).

The patch causes an incorrect lookup result:

```
namespace ns { struct Foo { };}

using ns::Foo;
void test() {
  struct Foo {
  } k; // the type of k refers to ns::Foo, rather than the local Foo!
}
```