KT-11645 generate backing field for 'private const val' in part implementation class

compiler/testData/codegen/box/multifileClasses/privateConstVal.kt

@@ -0,0 +1,23 @@
+// WITH_RUNTIME
+// FILE: foo.kt
+@file:JvmName("Util")
+@file:JvmMultifileClass
+package test
+
+private const val x = "O"
+
+fun foo() = x
+
+// FILE: bar.kt
+@file:JvmName("Util")
+@file:JvmMultifileClass
+package test
+
+private const val x = "K"
+
+fun bar() = x
+
+// FILE: test.kt
+package test
+
+fun box(): String = foo() + bar()