[FIR] Create string interpolating call even for single argument

Before this commit, questionable optimization existed which
unwrapped string interpolating call with single argument to this argument.
However, this led to source element loss and the necessity of sub-hacks.
In this commit we dropped this optimization (anyway user can remove
this single-expression string template in code if needed) to keep
source elements intact.

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 // WITH_RUNTIME
 

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

@@ -1,5 +1,4 @@
 // !LANGUAGE: +InlineClasses
-// IGNORE_BACKEND_FIR: JVM_IR
 
 // FILE: Z.kt
 inline class Z(val value: Int)

compiler/testData/codegen/bytecodeText/stringOperations/kt42457_old.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KOTLIN_CONFIGURATION_FLAGS: STRING_CONCAT=indy-with-constants
 // JVM_TARGET: 9
 // FILE: JavaClass.java

compiler/testData/codegen/bytecodeText/stringOperations/singleConcat.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun f(s: String) = "$s"
 
 fun g(s: String?) = "$s"

compiler/testData/ir/irText/expressions/stringTemplates.fir.txt

@@ -74,8 +74,9 @@ FILE fqName:<root> fileName:/stringTemplates.kt
   PROPERTY name:test7 visibility:public modality:FINAL [val]
     FIELD PROPERTY_BACKING_FIELD name:test7 type:kotlin.String visibility:private [final,static]
       EXPRESSION_BODY
-        CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.String' type=kotlin.String origin=null
-          $this: CALL 'public final fun <get-test1> (): kotlin.String declared in <root>' type=kotlin.String origin=GET_PROPERTY
+        STRING_CONCATENATION type=kotlin.String
+          CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.String' type=kotlin.String origin=null
+            $this: CALL 'public final fun <get-test1> (): kotlin.String declared in <root>' type=kotlin.String origin=GET_PROPERTY
     FUN DEFAULT_PROPERTY_ACCESSOR name:<get-test7> visibility:public modality:FINAL <> () returnType:kotlin.String
       correspondingProperty: PROPERTY name:test7 visibility:public modality:FINAL [val]
       BLOCK_BODY
@@ -84,8 +85,9 @@ FILE fqName:<root> fileName:/stringTemplates.kt
   PROPERTY name:test8 visibility:public modality:FINAL [val]
     FIELD PROPERTY_BACKING_FIELD name:test8 type:kotlin.String visibility:private [final,static]
       EXPRESSION_BODY
-        CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.String' type=kotlin.String origin=null
-          $this: CALL 'public final fun foo (): kotlin.String declared in <root>' type=kotlin.String origin=null
+        STRING_CONCATENATION type=kotlin.String
+          CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.String' type=kotlin.String origin=null
+            $this: CALL 'public final fun foo (): kotlin.String declared in <root>' type=kotlin.String origin=null
     FUN DEFAULT_PROPERTY_ACCESSOR name:<get-test8> visibility:public modality:FINAL <> () returnType:kotlin.String
       correspondingProperty: PROPERTY name:test8 visibility:public modality:FINAL [val]
       BLOCK_BODY
@@ -94,8 +96,9 @@ FILE fqName:<root> fileName:/stringTemplates.kt
   PROPERTY name:test9 visibility:public modality:FINAL [val]
     FIELD PROPERTY_BACKING_FIELD name:test9 type:kotlin.String visibility:private [final,static]
       EXPRESSION_BODY
-        CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.Int' type=kotlin.String origin=null
-          $this: CALL 'public final fun <get-x> (): kotlin.Int declared in <root>' type=kotlin.Int origin=GET_PROPERTY
+        STRING_CONCATENATION type=kotlin.String
+          CALL 'public open fun toString (): kotlin.String [fake_override] declared in kotlin.Int' type=kotlin.String origin=null
+            $this: CALL 'public final fun <get-x> (): kotlin.Int declared in <root>' type=kotlin.Int origin=GET_PROPERTY
     FUN DEFAULT_PROPERTY_ACCESSOR name:<get-test9> visibility:public modality:FINAL <> () returnType:kotlin.String
       correspondingProperty: PROPERTY name:test9 visibility:public modality:FINAL [val]
       BLOCK_BODY