Make indy-with-constants default for -jvm-target 9+

#KT-42522 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/state/GenerationState.kt

@@ -204,7 +204,7 @@ class GenerationState private constructor(
     val target = configuration.get(JVMConfigurationKeys.JVM_TARGET) ?: JvmTarget.DEFAULT
     val runtimeStringConcat =
         if (target.majorVersion >= JvmTarget.JVM_9.majorVersion)
-            configuration.get(JVMConfigurationKeys.STRING_CONCAT) ?: JvmStringConcat.INLINE
+            configuration.get(JVMConfigurationKeys.STRING_CONCAT) ?: JvmStringConcat.INDY_WITH_CONSTANTS
         else JvmStringConcat.INLINE
 
     val samConversionsScheme = run {

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -369,7 +369,9 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
         description = """Select code generation scheme for string concatenation.
 -Xstring-concat=indy-with-constants   Concatenate strings using `invokedynamic` `makeConcatWithConstants`. Requires `-jvm-target 9` or greater.
 -Xstring-concat=indy                Concatenate strings using `invokedynamic` `makeConcat`. Requires `-jvm-target 9` or greater.
--Xstring-concat=inline              Concatenate strings using `StringBuilder`"""
+-Xstring-concat=inline              Concatenate strings using `StringBuilder`
+default: `indy-with-constants` for JVM target 9 or greater, `inline` otherwise"""
+
     )
     var stringConcat: String? by NullableStringFreezableVar(JvmStringConcat.INLINE.description)
 

compiler/testData/cli/jvm/extraHelp.out

@@ -118,6 +118,7 @@ where advanced options include:
                              -Xstring-concat=indy-with-constants   Concatenate strings using `invokedynamic` `makeConcatWithConstants`. Requires `-jvm-target 9` or greater.
                              -Xstring-concat=indy                Concatenate strings using `invokedynamic` `makeConcat`. Requires `-jvm-target 9` or greater.
                              -Xstring-concat=inline              Concatenate strings using `StringBuilder`
+                             default: `indy-with-constants` for JVM target 9 or greater, `inline` otherwise
   -Xsupport-compatqual-checker-framework-annotations=enable|disable
                              Specify behavior for Checker Framework compatqual annotations (NullableDecl/NonNullDecl).
                              Default value is 'enable'