[clang][Sema] Provide source range to several Wunused warnings

When the diagnosed function/variable is a template specialization, the source range covers the specialization arguments.
e.g.
```
warning: unused function 'func<int>' [-Wunused-function]
template <> int func<int> () {}
                ^~~~~~~~~
```
This comes in line with the printed text in the warning message. In the above case, `func<int>`

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D152707