Fix case-insensitive character-wise comparison KT-45496

- Step 1: add failing tests
- Step 2: fix common case insensitive Char.equals
- Step 3: fix case insensitive String.equals in K/JS
- Step 4: enable unicode case folding in K/JS Regexes and
  string replacement (KT-45928)
- Step 5: fix case insensitive char comparison in K/N in String functions
  String.replace, equals, compareTo with ignoreCase

libraries/stdlib/test/text/StringTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -847,7 +847,7 @@ class StringTest {
             val result = v1.compareTo(v2, ignoreCase = ignoreCase).sign
             assertEquals(expectedResult, result, "Comparing '$v1' with '$v2', ignoreCase = $ignoreCase")
             if (expectedResult == 0)
-                assertTrue(v1.equals(v2, ignoreCase = ignoreCase))
+                assertTrue(v1.equals(v2, ignoreCase = ignoreCase), "'$v1'=='$v2' should be consistent with ordering, ignoreCase = $ignoreCase")
             if (!ignoreCase)
                 assertEquals(v1.compareTo(v2).sign, result)
         }
@@ -886,6 +886,16 @@ class StringTest {
                 assertCompareResult(EQ, item1, item2, ignoreCase = true)
             }
         }
+
+        CharTest.equalIgnoreCaseGroups
+            .filterNot { "i" in it }
+            .forEach { equalGroup ->
+                for (char1 in equalGroup) {
+                    for (char2 in equalGroup) {
+                        assertCompareResult(EQ, char1.toString(), char2.toString(), ignoreCase = true)
+                    }
+                }
+            }
     }
 
 
@@ -915,12 +925,14 @@ class StringTest {
                 assertEquals(expectAllReplaced, chars.replace(c, '_', ignoreCase = true), "$message, ignoreCase")
                 assertEquals(expectOneReplaced, chars.replace(c.toString(), "_"), "$message, as string")
                 assertEquals(expectAllReplaced, chars.replace(c.toString(), "_", ignoreCase = true), "$message, as string, ignoreCase")
+                assertEquals(expectOneReplaced, chars.replace(Regex(Regex.escape(c.toString())), "_"), "$message, as regex")
+                assertEquals(expectAllReplaced, chars.replace(Regex(Regex.escape(c.toString()), RegexOption.IGNORE_CASE), "_"), "$message, as regex, ignoreCase")
             }
         }
 
-        testIgnoreCase("üÜ")
-        testIgnoreCase("öÖ")
-        testIgnoreCase("äÄ")
+        CharTest.equalIgnoreCaseGroups
+            .filterNot { "i" in it } // not supported by JS
+            .forEach { testIgnoreCase(it) }
     }
 
     @Test fun replaceFirst() {