Fix a bug that cause the parser to not terminate if we recoverd to a case item

Author: ahoppen

Sources/SwiftParser/Expressions.swift

@@ -2253,6 +2253,9 @@ extension Parser {
       } else if allowStandaloneStmtRecovery && (self.atStartOfExpression() || self.atStartOfStatement() || self.atStartOfDeclaration()) {
         // Synthesize a label for the stamenent or declaration that isn't coverd by a case right now.
         let statements = parseSwitchCaseBody()
+        if statements.isEmpty {
+          break
+        }
         elements.append(
           .switchCase(
             RawSwitchCaseSyntax(

Tests/SwiftParserTest/ExpressionTests.swift

@@ -1641,4 +1641,15 @@ final class StatementExpressionTests: XCTestCase {
       ]
     )
   }
+
+  func testStandaloneAtCaseInSwitch() {
+    AssertParse(
+      """
+      switch x {
+        1️⃣@case
+      }
+      """,
+      diagnostics: [DiagnosticSpec(message: "unexpected code '@case' in 'switch' statement")]
+    )
+  }
 }