Move Spanned.

It's defined in `rustc_span::source_map` which doesn't make any sense
because it has nothing to do with source maps. This commit moves it to
the crate root, a more sensible spot for something this basic.

Author: nnethercote

src/context.rs

@@ -21,8 +21,7 @@ use rustc_middle::ty::{self, ExistentialTraitRef, Instance, Ty, TyCtxt};
 use rustc_session::Session;
 #[cfg(feature = "master")]
 use rustc_session::config::DebugInfo;
-use rustc_span::source_map::respan;
-use rustc_span::{DUMMY_SP, Span};
+use rustc_span::{DUMMY_SP, Span, respan};
 use rustc_target::spec::{HasTargetSpec, HasX86AbiOpt, Target, TlsModel, X86Abi};
 
 #[cfg(feature = "master")]