Generate specialized 'toString' for inline classes when possible

#KT-25613

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

@@ -0,0 +1,46 @@
+// !LANGUAGE: +InlineClasses
+// IGNORE_BACKEND: JVM_IR
+// WITH_RUNTIME
+
+import kotlin.test.*
+
+inline class Z(val value: Int)
+
+fun test1_1(z: Z) = "$z"
+fun test1_2(z: Z) = "$z$z"
+fun test1_many(z: Z) = "$z $z $z"
+fun test1_concat1(z: Z) = "-" + z
+fun test1_concat2(z: Z) = "$z" + z
+fun test1_concat3(z: Z) = "-" + z + z
+
+fun test2_1(z: Z?) = "$z"
+fun test2_2(z: Z?) = "$z$z"
+fun test2_many(z: Z?) = "$z $z $z"
+fun test2_concat1(z: Z?) = "-" + z
+fun test2_concat2(z: Z?) = "$z" + z
+fun test2_concat3(z: Z?) = "-" + z + z
+
+fun box(): String {
+    assertEquals("Z(value=42)", test1_1(Z(42)))
+    assertEquals("Z(value=42)Z(value=42)", test1_2(Z(42)))
+    assertEquals("Z(value=42) Z(value=42) Z(value=42)", test1_many(Z(42)))
+    assertEquals("-Z(value=42)", test1_concat1(Z(42)))
+    assertEquals("Z(value=42)Z(value=42)", test1_concat2(Z(42)))
+    assertEquals("-Z(value=42)Z(value=42)", test1_concat3(Z(42)))
+
+    assertEquals("null", test2_1(null))
+    assertEquals("nullnull", test2_2(null))
+    assertEquals("null null null", test2_many(null))
+    assertEquals("-null", test2_concat1(null))
+    assertEquals("nullnull", test2_concat2(null))
+    assertEquals("-nullnull", test2_concat3(null))
+
+    assertEquals("Z(value=42)", test2_1(Z(42)))
+    assertEquals("Z(value=42)Z(value=42)", test2_2(Z(42)))
+    assertEquals("Z(value=42) Z(value=42) Z(value=42)", test2_many(Z(42)))
+    assertEquals("-Z(value=42)", test2_concat1(Z(42)))
+    assertEquals("Z(value=42)Z(value=42)", test2_concat2(Z(42)))
+    assertEquals("-Z(value=42)Z(value=42)", test2_concat3(Z(42)))
+
+    return "OK"
+}