Test new @_expose(!Cxx) attribute syntax

Seems to work out of the box

Author: Gabor Horvath

Tests/SwiftParserTest/AttributeTests.swift

@@ -579,6 +579,12 @@ final class AttributeTests: ParserTestCase {
       """
     )
 
+    assertParse(
+      """
+      @_expose(!Cxx) func foo() {}
+      """
+    )
+
     assertParse(
       """
       @_expose(Cxx, "baz") func foo() {}