Merge #1049

Merge #1049

1049: Add better restrictions around semicolons in statements r=CohenArthur a=CohenArthur

When parsing macro invocations, rustc does not actually consume the
statement's trailing semicolon.

Let's take the following example:
```rust
macro_rules! one_stmt {
    ($s:stmt) => {};
}

macro_rules! one_or_more_stmt {
    ($($s:stmt)*) => {};
}

one_stmt!(let a = 1);
one_stmt!(let b = 2;); // error

one_or_more_stmt!(;); // valid
one_or_more_stmt!(let a = 15;); // valid, two statements!
one_or_more_stmt!(let a = 15 let b = 13); // valid, two statements again
```

A semicolon can count as a valid empty statement, but cannot be part of
a statement (in macro invocations). This commit adds more restrictions
that allow the parser to not always expect a semicolon token after the
statement. Furthermore, this fixes a test that was previously accepted
by the compiler but not by rustc.

Fixes #1046

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>