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

[japaric]

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]

---

Rebased version of #18958
r? @alexcrichton
cc @pcwalton

[nikomatsakis]

💃 I was wondering what would become of this PR