Raw FIR: introduce string concatenation call

This removes some 'plus' calls to be resolved

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt

@@ -304,14 +304,13 @@ internal fun Array<KtStringTemplateEntry>.toInterpolatingCall(
         }
         result = when {
             result == null -> nextArgument
-            callCreated && result is FirFunctionCallImpl -> result.apply {
+            callCreated && result is FirStringConcatenationCallImpl -> result.apply {
                 arguments += nextArgument
             }
             else -> {
                 callCreated = true
-                FirFunctionCallImpl(session, base).apply {
-                    calleeReference = FirSimpleNamedReference(session, base, OperatorNameConventions.PLUS)
-                    explicitReceiver = result
+                FirStringConcatenationCallImpl(session, base).apply {
+                    arguments += result!!
                     arguments += nextArgument
                 }
             }

compiler/fir/psi2fir/testData/rawBuilder/expressions/for.txt

@@ -43,7 +43,7 @@ FILE: for.kt
             lval <destruct>: <implicit> = <iterator>#.next#()
             lval x: <implicit> = <destruct>#.component1()
             lval y: <implicit> = <destruct>#.component2()
-            println#(String(x = ).plus#(x#, String( y = ), y#))
+            println#(<strcat>(String(x = ), x#, String( y = ), y#))
         }
 
     }