librustc: Always parse macro!()/macro![] as expressions if not

followed by a semicolon.

This allows code like `vec![1i, 2, 3].len();` to work.

This breaks code that uses macros as statements without putting
semicolons after them, such as:

    fn main() {
        ...
        assert!(a == b)
        assert!(c == d)
        println(...);
    }

It also breaks code that uses macros as items without semicolons:

    local_data_key!(foo)

    fn main() {
        println("hello world")
    }

Add semicolons to fix this code. Those two examples can be fixed as
follows:

    fn main() {
        ...
        assert!(a == b);
        assert!(c == d);
        println(...);
    }

    local_data_key!(foo);

    fn main() {
        println("hello world")
    }

RFC #378.

Closes #18635.

[breaking-change]

src/librustc_back/svh.rs

@@ -340,14 +340,17 @@ mod svh_visitor {
                 // expensive; a direct content-based hash on token
                 // trees might be faster. Implementing this is far
                 // easier in short term.
-                let macro_defn_as_string =
-                    pprust::to_string(|pp_state| pp_state.print_mac(macro));
+                let macro_defn_as_string = pprust::to_string(|pp_state| {
+                    pp_state.print_mac(macro, token::Paren)
+                });
                 macro_defn_as_string.hash(self.st);
             } else {
                 // It is not possible to observe any kind of macro
                 // invocation at this stage except `macro_rules!`.
                 panic!("reached macro somehow: {}",
-                      pprust::to_string(|pp_state| pp_state.print_mac(macro)));
+                      pprust::to_string(|pp_state| {
+                          pp_state.print_mac(macro, token::Paren)
+                      }));
             }
 
             visit::walk_mac(self, macro);