Auto merge of #115371 - matthewjasper:if-let-guard-parsing, r=cjgillot

Make if let guard parsing consistent with normal guards

- Add tests that struct expressions are not allowed in `if let` and `while let` (no change, consistent with `if` and `while`)
- Allow struct expressions in `if let` guards (consistent with `if` guards).

r? `@cjgillot`

Closes #93817
cc #51114

Author: bors