Deprecate String.concat with error severity.

Fix concat j2k conversion.
Add expected deprecation errors for Java String constructors that must be converted to special Kotlin String methods.

libraries/stdlib/src/kotlin/text/Indent.kt

@@ -105,7 +105,7 @@ private fun String.indentWidth(): Int = indexOfFirst { !it.isWhitespace() }.let
 
 private fun getIndentFunction(indent: String) = when {
     indent.isEmpty() -> { line: String -> line }
-    else -> { line: String -> indent.concat(line) }
+    else -> { line: String -> indent + line }
 }
 
 private inline fun List<String>.reindent(resultSizeEstimate: Int, indentAddFunction: (String) -> String, indentCutFunction: (String) -> String?): String {

libraries/stdlib/src/kotlin/text/StringsJVM.kt

@@ -277,7 +277,7 @@ public fun String.compareTo(other: String, ignoreCase: Boolean = false): Int {
 /**
  * Returns a new string obtained by concatenating this string and the specified string.
  */
-// TODO: Deprecated in favor of operator plus, when it would be as efficient as concat
+@Deprecated("Use this + other, eventually it will be optimized as concat.", ReplaceWith("this + other"), level = DeprecationLevel.ERROR)
 public fun String.concat(other: String): String = (this as java.lang.String).concat(other)
 
 /**