Unified: Consolidate conditional pattern rules

I'm pleasantly surprised that the new syntax for template choices
supports having a Rust block as an alternative. We may need to annotate
these with the "return type" at some point, in order to make type
checking work out, but that can wait.

Author: tausbn

unified/extractor/src/languages/swift/swift.rs

@@ -922,13 +922,23 @@ fn translation_rules() -> Vec<Rule<SwiftContext>> {
         ),
         // A single case item unwraps to its pattern, possibly boxed in conditional_pattern
         rule!(
-            (switchCaseItem
-                pattern: @p
-                whereClause: (whereClause condition: @cond))
+            [
+                (switchCaseItem
+                    pattern: @pattern
+                    whereClause: (whereClause condition: @condition)?)
+                (catchItem
+                    pattern: @pattern
+                    whereClause: (whereClause condition: @condition)?)
+            ]
             =>
-            (conditional_pattern pattern: {p} condition: {cond})
+            [
+                (conditional_pattern
+                    pattern: {pattern}
+                    condition: {condition})
+                    if { condition.is_some() }
+                {pattern}
+            ]
         ),
-        rule!((switchCaseItem pattern: @p) => pattern { p }),
         // A pattern-matching condition (`if case let x = e`, `if case .foo(let x)
         // = e`) becomes a `pattern_guard_expr`: the matched pattern and the
         // scrutinee value are translated recursively.
@@ -1093,18 +1103,6 @@ fn translation_rules() -> Vec<Rule<SwiftContext>> {
                 body: {body}
                 catch_clause: {catches})
         ),
-        rule!(
-            (catchItem
-                pattern: @pattern
-                whereClause: (whereClause condition: @guard))
-            =>
-            (conditional_pattern pattern: {pattern} condition: {guard})
-        ),
-        rule!(
-            (catchItem pattern: @pattern)
-            =>
-            pattern {pattern}
-        ),
         // Catch block with one or more patterns (which have been translated by the catchItem rules)
         rule!(
             (catchClause