Introduce BareFnTy::decl_span and fix generics span.

compiler/rustc_ast/src/ast.rs

@@ -1976,6 +1976,8 @@ pub struct BareFnTy {
     pub ext: Extern,
     pub generic_params: Vec<GenericParam>,
     pub decl: P<FnDecl>,
+    /// Span of the `fn(...) -> ...` part.
+    pub decl_span: Span,
 }
 
 /// The various kinds of type recognized by the compiler.