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/libstd/sys/unix/process.rs

@@ -28,7 +28,7 @@ use sys_common::{AsInner, mkerr_libc, timeout};
 
 pub use sys_common::ProcessConfig;
 
-helper_init!(static HELPER: Helper<Req>)
+helper_init! { static HELPER: Helper<Req> }
 
 /// The unique id of the process (this should never be negative).
 pub struct Process {