Parser: recover on missing 'when' conditions

[auduchinok]

Fixes parsing of missing when clause expressions, like the following:

match () with
| _ when -> ()

match () with
| a when
| b -> ()

[github-actions]

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: modifies parser grammar and CheckDeclarations

Generated by PR Tooling Safety Check · opus46 5.3M · ◷