Addresses SR-2605.
If you're refactoring a `guard` into an `if`, it's easy to
accidentally end up with invalid code like this in the transition:
```
if condition else {
}
```
The parser rightly complains about the `else`, but has poor recovery
afterward, interpreting the following brace as a closure, which may in
turn lead to other unhelpful errors after:
```
error: expected '{' after 'if' condition
if condition else {
^
error: braced block of statements is an unused closure
if condition else {
^
```
Improve the diagnostics to instead explicitly detect this pattern and
mark it as an error.
