Returns a `&core::panic::Location` corresponding to where it was
called, also making `Location` a lang item.

The ICE stderr isn't normalizing correctly on some builders.

Also add a missing terminal newline to a test.

This ICEs in MIR currently, which I think is to be expected since none of the MIR plumbing is set up. I added a test which confirms that the shim is being used for reifying a track_caller function.

Prevents number collision with another approved PR.

- The attribute is behind a feature gate.
- Error if both #[naked] and #[track_caller] are applied to the same function.
- Error if #[track_caller] is applied to a non-function item.
- Error if ABI is not "rust"
- Error if #[track_caller] is applied to a trait function.

Error codes and descriptions are pending.