Bring back suggestion for splitting <- into < -

Closes #62632

src/test/ui/obsolete-in-place/bad.rs

@@ -2,7 +2,7 @@
 
 fn foo() {
     let (x, y) = (0, 0);
-    x <- y; //~ ERROR expected one of
+    x <- y; //~ ERROR unexpected token: `<-`
 }
 
 fn main() {

src/test/ui/obsolete-in-place/bad.stderr

@@ -1,8 +1,12 @@
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `<-`
+error: unexpected token: `<-`
   --> $DIR/bad.rs:5:7
    |
 LL |     x <- y;
-   |       ^^ expected one of 8 possible tokens here
+   |       ^^
+help: if you meant to write a comparison against a negative value, add a space in between `<` and `-`
+   |
+LL |     x < - y;
+   |       ^^^
 
 error: expected expression, found keyword `in`
   --> $DIR/bad.rs:10:5

src/test/ui/placement-syntax.rs

@@ -1,6 +1,6 @@
 fn main() {
     let x = -5;
-    if x<-1 { //~ ERROR expected `{`, found `<-`
+    if x<-1 { //~ ERROR unexpected token: `<-`
         println!("ok");
     }
 }

src/test/ui/placement-syntax.stderr

@@ -1,10 +1,12 @@
-error: expected `{`, found `<-`
+error: unexpected token: `<-`
   --> $DIR/placement-syntax.rs:3:9
    |
 LL |     if x<-1 {
-   |     --  ^^ expected `{`
-   |     |
-   |     this `if` statement has a condition, but no block
+   |         ^^
+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