stdlib: Always find an empty substring

This patch fixes the following tests:
    external_stdlib_text_StringTest/replace.kt
    external_stdlib_text_StringTest/replaceDelimited.kt
    external_stdlib_text_StringTest/replaceFirst.kt

runtime/src/main/cpp/KString.cpp

@@ -1060,6 +1060,10 @@ KInt Kotlin_String_indexOfString(KString thiz, KString other, KInt fromIndex) {
       other->count_ > thiz->count_ - fromIndex) {
     return -1;
   }
+  // An empty string can be always found.
+  if (other->count_ == 0) {
+    return fromIndex;
+  }
   KInt count = thiz->count_;
   const KChar* thizRaw = CharArrayAddressOfElementAt(thiz, fromIndex);
   const KChar* otherRaw = CharArrayAddressOfElementAt(other, 0);

runtime/src/main/kotlin/kotlin/text/Strings.kt

@@ -640,10 +640,6 @@ public fun String.replaceBeforeLast(delimiter: String, replacement: String, miss
     return if (index == -1) missingDelimiterValue else replaceRange(0, index, replacement)
 }
 
-
-// public fun String.replace(oldChar: Char, newChar: Char, ignoreCase: Boolean): String // JVM- and JS-specific
-// public fun String.replace(oldValue: String, newValue: String, ignoreCase: Boolean): String // JVM- and JS-specific
-
 /**
  * Returns a new string obtained by replacing each substring of this char sequence that matches the given regular expression
  * with the given [replacement].