Suggest correct comparison against negative literal

When parsing as emplacement syntax (`x<-1`), suggest the correct syntax
for comparison against a negative value (`x< -1`).

src/test/ui/suggestions/placement-syntax.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let x = -5;
+    if x<-1 {
+    //~^ ERROR emplacement syntax is obsolete
+        println!("ok");
+    }
+}

src/test/ui/suggestions/placement-syntax.stderr

@@ -0,0 +1,14 @@
+error: emplacement syntax is obsolete (for now, anyway)
+  --> $DIR/placement-syntax.rs:13:8
+   |
+LL |     if x<-1 {
+   |        ^^^^
+   |
+   = note: for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>
+help: if you meant to write a comparison against a negative value, add a space in between `<` and `-`
+   |
+LL |     if x< -1 {
+   |         ^^^
+
+error: aborting due to previous error
+