Support parenthesized paths Foo(A,B) -> C that expand to Foo<(A,B),C>. These paths also bind anonymous regions (or will, once HRTB is fully working).

Fixes #18423.

src/libsyntax/test.rs

@@ -453,8 +453,7 @@ fn path_node(ids: Vec<ast::Ident> ) -> ast::Path {
         global: false,
         segments: ids.into_iter().map(|identifier| ast::PathSegment {
             identifier: identifier,
-            lifetimes: Vec::new(),
-            types: OwnedSlice::empty(),
+            parameters: ast::PathParameters::none(),
         }).collect()
     }
 }