[Stubs] Fix stub inconsistency on unnamed setter parameters

If an unused setter parameter was replaced with an underscore ('_'),
it had a special name in the stub ('<anonymous parameter 0>'). The text
version of the stub inserted it as is, leading to an extra
'PsiErrorElement'.

^KTIJ-28194 Fixed

core/compiler.common/src/org/jetbrains/kotlin/name/SpecialNames.kt

@@ -88,6 +88,18 @@ object SpecialNames {
         return Name.special("<index_$idx>")
     }
 
+    private const val ANONYMOUS_PARAMETER_NAME_PREFIX = "anonymous parameter"
+
+    @JvmStatic
+    fun anonymousParameterName(index: Int): Name {
+        return Name.special("<$ANONYMOUS_PARAMETER_NAME_PREFIX $index>")
+    }
+
+    @JvmStatic
+    fun isAnonymousParameterName(name: Name): Boolean {
+        return name.isSpecial && name.asStringStripSpecialMarkers().startsWith(ANONYMOUS_PARAMETER_NAME_PREFIX)
+    }
+
     @JvmStatic
     fun safeIdentifier(name: Name?): Name {
         return if (name != null && !name.isSpecial) name else SAFE_IDENTIFIER_FOR_NO_NAME