Improve error message on a conflicting explicit impl

[dtolnay]

Before:

error[E0119]: conflicting implementations of trait `cxx::private::UniquePtrTarget` for type `here::C`:
  --> $DIR/unique_ptr_twice.rs:10:1
   |
1  | #[cxx::bridge]
   | -------------- first implementation here
...
10 | #[cxx::bridge]
   | ^^^^^^^^^^^^^^ conflicting implementation for `here::C`
   |
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

After:

error[E0119]: conflicting implementations of trait `cxx::private::UniquePtrTarget` for type `here::C`:
  --> $DIR/unique_ptr_twice.rs:16:5
   |
7  |     impl UniquePtr<C> {}
   |     ----------------- first implementation here
...
16 |     impl UniquePtr<C> {}
   |     ^^^^^^^^^^^^^^^^^ conflicting implementation for `here::C`

Cleanup of #336.