[SwiftParser] Handle extraneous case keyword after comma in switch case.

Sources/SwiftParser/Expressions.swift

@@ -2361,17 +2361,28 @@ extension Parser {
     do {
       var keepGoing: RawTokenSyntax? = nil
       var loopProgress = LoopProgressCondition()
+      var unexpectedPrePatternCase: RawUnexpectedNodesSyntax? = nil
       repeat {
         let (pattern, whereClause) = self.parseGuardedCasePattern()
         keepGoing = self.consume(if: .comma)
         caseItems.append(
           RawSwitchCaseItemSyntax(
+            unexpectedPrePatternCase,
             pattern: pattern,
             whereClause: whereClause,
             trailingComma: keepGoing,
             arena: self.arena
           )
         )
+
+        if keepGoing != nil, let caseToken = self.consume(if: .keyword(.case)) {
+          unexpectedPrePatternCase = RawUnexpectedNodesSyntax(
+            elements: [RawSyntax(caseToken)],
+            arena: self.arena
+          )
+        } else {
+          unexpectedPrePatternCase = nil
+        }
       } while keepGoing != nil && self.hasProgressed(&loopProgress)
     }
     let (unexpectedBeforeColon, colon) = self.expect(.colon)

Tests/SwiftParserTest/translated/SwitchTests.swift

@@ -1351,4 +1351,17 @@ final class SwitchTests: ParserTestCase {
         """
     )
   }
+
+  func testSwitch84() {
+    assertParse(
+      """
+      switch x {
+      case 1, 1️⃣case 2, 3:
+      }
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "unexpected 'case' keyword in switch case")
+      ]
+    )
+  }
 }