ICU4J: Allow any type of expression in an unsupported statement, per grammar

Author: catamorphism

icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java

@@ -770,8 +770,7 @@ private MFDataModel.Declaration getDeclaration() throws MFParseException {
                         input.backup(2);
                     }
                     expression = getPlaceholder();
-                    // This also covers != null
-                    if (expression instanceof MFDataModel.VariableExpression) {
+                    if (expression != null) {
                         expressions.add(expression);
                     } else {
                         break;

testdata/message2/reserved-syntax-2.json

@@ -9,7 +9,8 @@
       { "src" : "\\\\{   ?   󗟋   𫓜|@}   |   \\}   @𠟅򑚎𥪙𽧫=||   @򒘒򳷦㥞򉊷=$򸚶񽱆񅗽񤕞   @𰺱:񎫛񢶛򶈎񄮒}", "ignoreCpp": "This would be an error in ICU4C" },
       { "src" : "{  $iFN  ^  @USh  =$u  @l}", "ignoreCpp": "This would be an error in ICU4C" },
       { "src" : ".local $dS ={ $p4 ^ |.| \\\\ @g:FV = $kd}{{@}}", "ignoreCpp": "This would be an error in ICU4C" },
-      { "src" : ".D. \\\\ ||{1}{{}} ", "ignoreCpp": "This would be an error in ICU4C" }
+      { "src" : ".D. \\\\ ||{1}{{}} ", "ignoreCpp": "This would be an error in ICU4C" },
+      { "src" : ".cIT  ||{|@|  %  \\}  }  {  *󔜫񥘃󸇀  }{{}}", "ignoreCpp": "This would be an error in ICU4C" }
   ]
 }