auto merge of #12366 : aepsil0n/rust/feature/unnecessary_parens_around_assigned_values, r=alexcrichton

Fixes #12350.

Parentheses around assignment statements such as

```rust
let mut a = (0);
a = (1);
a += (2);
```

are not necessary and therefore an unnecessary_parens warning is raised when
statements like this occur.

NOTE: In `let` declarations this does not work as intended. Is it possible that they do not count as assignment expressions (`ExprAssign`)? (edit: this is fixed by now)

Furthermore, there are some cases that I fixed in the rest of the code, where parentheses could potentially enhance readability. Compare these lines:

```rust
a = b == c;
a = (b == c);
```

Thus, after having worked on this I'm not entirely sure, whether we should go through with this patch or not. Probably a matter of debate. ;)

src/test/compile-fail/lint-unnecessary-parens.rs

@@ -22,4 +22,7 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
+    let mut _a = (0); //~ ERROR unnecessary parentheses around assigned value
+    _a = (0); //~ ERROR unnecessary parentheses around assigned value
+    _a += (1); //~ ERROR unnecessary parentheses around assigned value
 }