Add "reserved literals" to the list of reserved names for Java.

The identifiers `true`, `false`, and `null` are effectively reserved words in Java, although for some reason they are listed separately from the "keywords" in the Java Language Specification.

This doesn't matter for regular fields, because a proto field called `true` will be accessed with `getTrue` and `setTrue`. But for extensions, the generated Java code will have a public static field whose name is the same as the name of the extension field, with `_` appended if the name is a reserved word. Previously there was no `_` for `true` etc, so the generated code would not compile.

PiperOrigin-RevId: 631599695

src/google/protobuf/compiler/java/names.cc

@@ -39,16 +39,17 @@ const char* DefaultPackage(Options options) {
 bool IsReservedName(absl::string_view name) {
   static const auto& kReservedNames =
       *new absl::flat_hash_set<absl::string_view>({
-          "abstract",   "assert",       "boolean",   "break",      "byte",
-          "case",       "catch",        "char",      "class",      "const",
-          "continue",   "default",      "do",        "double",     "else",
-          "enum",       "extends",      "final",     "finally",    "float",
-          "for",        "goto",         "if",        "implements", "import",
-          "instanceof", "int",          "interface", "long",       "native",
-          "new",        "package",      "private",   "protected",  "public",
-          "return",     "short",        "static",    "strictfp",   "super",
-          "switch",     "synchronized", "this",      "throw",      "throws",
-          "transient",  "try",          "void",      "volatile",   "while",
+          "abstract", "assert",     "boolean",      "break",     "byte",
+          "case",     "catch",      "char",         "class",     "const",
+          "continue", "default",    "do",           "double",    "else",
+          "enum",     "extends",    "false",        "final",     "finally",
+          "float",    "for",        "goto",         "if",        "implements",
+          "import",   "instanceof", "int",          "interface", "long",
+          "native",   "new",        "package",      "private",   "protected",
+          "public",   "return",     "short",        "static",    "strictfp",
+          "super",    "switch",     "synchronized", "this",      "throw",
+          "throws",   "transient",  "true",         "try",       "void",
+          "volatile", "while",
       });
   return kReservedNames.contains(name);
 }