[clang] Fix the location of UsingTypeLoc.

It is revealed by the https://reviews.llvm.org/D141280.

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

// Before the fix, the Location of UsingTypeLoc Foo points to the
token "class", slection on ^Foo will result in the VarDecl abc.
class Foo abc;

```

Differential Revision: https://reviews.llvm.org/D142125