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/test/run-pass/syntax-extension-source-utils.rs

@@ -21,7 +21,7 @@ pub mod m1 {
     }
 }
 
-macro_rules! indirect_line( () => ( line!() ) )
+macro_rules! indirect_line( () => ( line!() ) );
 
 pub fn main() {
     assert_eq!(line!(), 27);