[FIR] Add toString() call to expressions in string templates

compiler/testData/codegen/box/strings/kt3652.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box(): String {
  var a = 'a'
 

compiler/testData/codegen/box/strings/kt881.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box() : String {
   val b = 1+1
   if ("$b" != "2") return "fail"

compiler/testData/codegen/box/strings/singleConcatNullable.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun f(s: String?): String {
     return "$s"
 }
@@ -6,4 +5,4 @@ fun f(s: String?): String {
 fun box(): String {
     if (f(null) != "null") return "Fail"
     return "OK"
-}
+}