Add "Too long character literal -> string" fix to ILLEGAL_ESCAPE error

So #KT-23788 Fixed

idea/src/org/jetbrains/kotlin/idea/quickfix/QuickFixRegistrar.kt

@@ -484,6 +484,7 @@ class QuickFixRegistrar : QuickFixContributor {
         CANNOT_CHECK_FOR_ERASED.registerFactory(MakeTypeParameterReifiedAndFunctionInlineFix)
 
         TOO_MANY_CHARACTERS_IN_CHARACTER_LITERAL.registerFactory(TooLongCharLiteralToStringFix)
+        ILLEGAL_ESCAPE.registerFactory(TooLongCharLiteralToStringFix)
 
         UNUSED_VALUE.registerFactory(RemoveUnusedValueFix)
 

idea/src/org/jetbrains/kotlin/idea/quickfix/TooLongCharLiteralToStringFix.kt

@@ -34,7 +34,8 @@ class TooLongCharLiteralToStringFix(
 
     companion object Factory : KotlinSingleIntentionActionFactory() {
         override fun createAction(diagnostic: Diagnostic): IntentionAction? {
-            val element = Errors.TOO_MANY_CHARACTERS_IN_CHARACTER_LITERAL.cast(diagnostic).psiElement
+            val element = diagnostic.psiElement as? KtConstantExpression ?: return null
+            if (element.text == "'\\'") return null
             return TooLongCharLiteralToStringFix(element = element)
         }
     }

idea/testData/quickfix/tooLongCharLiteralToString/backslash.kt

@@ -0,0 +1,7 @@
+// "Convert too long character literal to string" "false"
+// ACTION: Introduce local variable
+// ERROR: Illegal escape: ''\''
+
+fun foo() {
+    '\'<caret>
+}

idea/testData/quickfix/tooLongCharLiteralToString/startWithBackslash.kt

@@ -0,0 +1,6 @@
+// "Convert too long character literal to string" "true"
+// ERROR: Illegal escape: '\ '
+
+fun foo() {
+    '\ bar<caret>'
+}

idea/testData/quickfix/tooLongCharLiteralToString/startWithBackslash.kt.after

@@ -0,0 +1,6 @@
+// "Convert too long character literal to string" "true"
+// ERROR: Illegal escape: '\ '
+
+fun foo() {
+    "\ bar"
+}

idea/tests/org/jetbrains/kotlin/idea/quickfix/QuickFixTestGenerated.java

@@ -10496,6 +10496,11 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
             KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/testData/quickfix/tooLongCharLiteralToString"), Pattern.compile("^([\\w\\-_]+)\\.kt$"), TargetBackend.ANY, true);
         }
 
+        @TestMetadata("backslash.kt")
+        public void testBackslash() throws Exception {
+            runTest("idea/testData/quickfix/tooLongCharLiteralToString/backslash.kt");
+        }
+
         @TestMetadata("backslashShouldNotBeEscaped.kt")
         public void testBackslashShouldNotBeEscaped() throws Exception {
             runTest("idea/testData/quickfix/tooLongCharLiteralToString/backslashShouldNotBeEscaped.kt");
@@ -10515,6 +10520,11 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
         public void testEcapedDoubleQuotesShouldNotBeEscaped() throws Exception {
             runTest("idea/testData/quickfix/tooLongCharLiteralToString/ecapedDoubleQuotesShouldNotBeEscaped.kt");
         }
+
+        @TestMetadata("startWithBackslash.kt")
+        public void testStartWithBackslash() throws Exception {
+            runTest("idea/testData/quickfix/tooLongCharLiteralToString/startWithBackslash.kt");
+        }
     }
 
     @TestMetadata("idea/testData/quickfix/typeAddition")