IR: use inlineClassRepresentation in getInlineClassUnderlyingType

Looking for the primary constructor manually doesn't work if it's
private in the other module on JVM, because private declarations are
skipped in IrLazyClass.

compiler/testData/codegen/box/inlineClasses/privateConstructorFunInterfaceMultiModule.kt

@@ -0,0 +1,23 @@
+// IGNORE_BACKEND: WASM
+// MODULE: lib
+// FILE: lib.kt
+
+inline class Z private constructor(private val value: Any?) {
+    fun result(): String = value as String
+
+    companion object {
+        fun create(value: Any?): Z = Z(value)
+    }
+}
+
+fun interface IFoo<T> {
+    fun foo(x: T): String
+}
+
+fun foo1(fs: IFoo<Z>) = fs.foo(Z.create("OK"))
+
+// MODULE: main(lib)
+// FILE: main.kt
+
+fun box(): String =
+    foo1 { it.result() }