unified: Add some corpus tests

Output has not been generated yet (for reasons)

unified/extractor/tests/corpus/swift/control-flow/switch-case-item-where-clauses.swift

@@ -0,0 +1,8 @@
+switch n {
+case let x where x > 0:
+  print("positive")
+case let y where y < 0, 0:
+  print("non-positive")
+default:
+  print("other")
+}

unified/extractor/tests/corpus/swift/optionals-and-errors/catch-where-clauses.swift

@@ -0,0 +1,7 @@
+do {
+  try foo()
+} catch let e where isNetworkError(e), let f where isTimeout(f) {
+  print("retry")
+} catch {
+  print("fallback")
+}