Add possibility to interpret and fold IrStringConcatenation expression

compiler/testData/codegen/box/involvesIrInterpreter/dumpIrAndCheck/stringConcatenation.kt

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM_IR
+// TARGET_BACKEND: NATIVE
+// TARGET_BACKEND: JS_IR
+// WITH_STDLIB
+
+const val simple = "O${'K'} ${1.toLong() + 2.5}"
+const val withInnerConcatenation = "1 ${"2 ${3} ${4} 5"} 6"
+const val withNull = "1 ${null}" // but `"1" + null` is invalid
+
+fun box(): String {
+    if (simple != "OK 3.5") return "Fail 1"
+    if (withInnerConcatenation != "1 2 3 4 5 6") return "Fail 2"
+    if (withNull != "1 null") return "Fail 3"
+
+    return "OK"
+}