[clang] Build UsingType for elaborated type specifiers.

Support building UsingType for elaborated type specifiers:

```
namespace ns { class Foo {}; }

using ns::Foo;

// The TypeLoc of `Foo` below should be a ElaboratedTypeLoc with an
// inner UsingTypeLoc rather than the underlying `CXXRecordTypeLoc`
class Foo foo;
```

Differential Revision: https://reviews.llvm.org/D141280