syntax: Allow semi tokens after macro ty/path

This commit expands the follow set of the `ty` and `path` macro fragments to
include the semicolon token as well. A semicolon is already allowed after these
tokens, so it's currently a little too restrictive to not have a semicolon
allowed. For example:

    extern {
        fn foo() -> i32; // semicolon after type
    }

    fn main() {
        struct Foo;

        Foo; // semicolon after path
    }

src/libsyntax/ext/tt/macro_rules.rs

@@ -501,7 +501,7 @@ fn is_in_follow(_: &ExtCtxt, tok: &Token, frag: &str) -> Result<bool, String> {
             },
             "path" | "ty" => {
                 match *tok {
-                    Comma | FatArrow | Colon | Eq | Gt => Ok(true),
+                    Comma | FatArrow | Colon | Eq | Gt | Semi => Ok(true),
                     Ident(i, _) if i.as_str() == "as" => Ok(true),
                     _ => Ok(false)
                 }