Fix NPE on .companionObjectInstance for built-in classes

#KT-13370 Fixed

compiler/testData/codegen/box/reflection/classes/companionObject.kt

@@ -25,5 +25,24 @@ fun box(): String {
 
     assertEquals(E.Companion, E::class.companionObjectInstance)
 
+    assertEquals(String, String::class.companionObjectInstance)
+    assertEquals(String, String.Companion::class.objectInstance)
+    assertEquals(Enum, Enum::class.companionObjectInstance)
+    assertEquals(Enum, Enum.Companion::class.objectInstance)
+    assertEquals(Double, Double::class.companionObjectInstance)
+    assertEquals(Double, Double.Companion::class.objectInstance)
+    assertEquals(Float, Float::class.companionObjectInstance)
+    assertEquals(Float, Float.Companion::class.objectInstance)
+    assertEquals(Int, Int::class.companionObjectInstance)
+    assertEquals(Int, Int.Companion::class.objectInstance)
+    assertEquals(Long, Long::class.companionObjectInstance)
+    assertEquals(Long, Long.Companion::class.objectInstance)
+    assertEquals(Short, Short::class.companionObjectInstance)
+    assertEquals(Short, Short.Companion::class.objectInstance)
+    assertEquals(Byte, Byte::class.companionObjectInstance)
+    assertEquals(Byte, Byte.Companion::class.objectInstance)
+    assertEquals(Char, Char::class.companionObjectInstance)
+    assertEquals(Char, Char.Companion::class.objectInstance)
+
     return "OK"
 }

compiler/testData/codegen/box/reflection/classes/objectInstance.kt

@@ -26,6 +26,8 @@ fun box(): String {
     assertEquals(3, B.Factory::class.objectInstance!!.foo())
 
     assertEquals(null, C::class.objectInstance)
+    assertEquals(null, String::class.objectInstance)
+    assertEquals(Unit, Unit::class.objectInstance)
 
     return "OK"
 }

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KClassImpl.kt

@@ -16,6 +16,7 @@
 
 package kotlin.reflect.jvm.internal
 
+import org.jetbrains.kotlin.builtins.CompanionObjectMapping
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.descriptors.annotations.Annotated
 import org.jetbrains.kotlin.incremental.components.NoLookupLocation
@@ -127,7 +128,7 @@ internal class KClassImpl<T : Any>(override val jClass: Class<T>) :
         val descriptor = descriptor
         if (descriptor.kind != ClassKind.OBJECT) return@lazy null
 
-        val field = if (descriptor.isCompanionObject) {
+        val field = if (descriptor.isCompanionObject && !CompanionObjectMapping.hasMappingToObject(descriptor)) {
             jClass.enclosingClass.getDeclaredField(descriptor.name.asString())
         }
         else {