[RFC 2091] Add #[track_caller] attribute.

- 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.

src/librustc/error_codes.rs

@@ -1640,6 +1640,16 @@ each method; it is not possible to annotate the entire impl with an `#[inline]`
 attribute.
 "##,
 
+E0900: r##"
+TODO: change error number
+TODO: track_caller: invalid syntax
+"##,
+
+E0901: r##"
+TODO: change error number
+TODO: track_caller: no naked functions
+"##,
+
 E0522: r##"
 The lang attribute is intended for marking special items that are built-in to
 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect