I initially added these as part of my TryAnalyzer rewrite to allow testing complicated `finally` types like this:
```
$foo = 1;
try {
$foo = 2;
} catch (Exception $_) {
$foo = 3;
} finally {
$bar = $foo;
/** @psalm-check-type-exact $bar = 1|2|3 */;
}
/** @psalm-check-type-exact $bar = 2|3 */;
```
Using the `'assertions'` in tests doesn't work since the type is different inside the `finally`.
I decided to extract the new annotation from the rest of my changes and do a separate pull request since I think others may find it useful, and it should be much easier to review than the entire TryAnalyzer rewrite.
