syntax: allow trace_macros! and log_syntax! in item position.

Previously

    trace_macros!(true)
    fn main() {}

would complain about `trace_macros` being an expression macro in item
position. This is a pointless limitation, because the macro is purely
compile-time, with no runtime effect. (And similarly for log_syntax.)

This also changes the behaviour of `trace_macros!` very slightly, it
used to be equivalent to

    macro_rules! trace_macros {
        (true $($_x: tt)*) => { true };
        (false $($_x: tt)*) => { false }
    }

I.e. you could invoke it with arbitrary trailing arguments, which were
ignored. It is changed to accept only exactly `true` or `false` (with no
trailing arguments) and expands to `()`.

src/libsyntax/ext/log_syntax.rs

@@ -10,11 +10,10 @@
 
 use ast;
 use codemap;
-use ext::base::*;
 use ext::base;
 use print;
 
-pub fn expand_syntax_ext(cx: &mut ExtCtxt,
+pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
                          sp: codemap::Span,
                          tt: &[ast::TokenTree])
                       -> base::MacResult {
@@ -23,13 +22,6 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt,
     println!("{}", print::pprust::tt_to_str(&ast::TTDelim(
                 @tt.iter().map(|x| (*x).clone()).collect())));
 
-    //trivial expression
-    MRExpr(@ast::Expr {
-        id: ast::DUMMY_NODE_ID,
-        node: ast::ExprLit(@codemap::Spanned {
-            node: ast::LitNil,
-            span: sp
-        }),
-        span: sp,
-    })
+    // any so that `log_syntax` can be invoked as an expression and item.
+    base::MacResult::dummy_any(sp)
 }