Rename syntax to rustc_ast in source code

src/test/ui/regions/regions-trait-object-1.rs

@@ -1,7 +1,7 @@
 // run-pass
 // This is a regression test for something that only came up while
 // attempting to bootstrap librustc_ast; it is adapted from
-// `syntax::ext::tt::generic_extension`.
+// `rustc_ast::ext::tt::generic_extension`.
 
 
 pub struct E<'a> {

src/test/ui/unboxed-closures/unboxed-closures-unique-type-id.rs

@@ -4,7 +4,7 @@
 //
 //    error: internal compiler error: get_unique_type_id_of_type() -
 //    unexpected type: closure,
-//    Closure(syntax::ast::DefId{krate: 0, node: 66},
+//    Closure(rustc_ast::ast::DefId{krate: 0, node: 66},
 //    ReScope(63))
 //
 // This is a regression test for issue #17021.