Raw FIR: introduce string concatenation call

This removes some 'plus' calls to be resolved

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -80,6 +80,8 @@ internal class Fir2IrVisitor(
 
     private val booleanType = FirImplicitBooleanTypeRef(session, null).toIrType(session, declarationStorage)
 
+    private val stringType = FirImplicitStringTypeRef(session, null).toIrType(session, declarationStorage)
+
     private fun ModuleDescriptor.findPackageFragmentForFile(file: FirFile): PackageFragmentDescriptor =
         getPackage(file.packageFqName).fragments.first()
 
@@ -1070,6 +1072,15 @@ internal class Fir2IrVisitor(
         }
     }
 
+    override fun visitStringConcatenationCall(stringConcatenationCall: FirStringConcatenationCall, data: Any?): IrElement {
+        return stringConcatenationCall.convertWithOffsets { startOffset, endOffset ->
+            IrStringConcatenationImpl(
+                startOffset, endOffset, stringType,
+                stringConcatenationCall.arguments.map { it.toIrExpression() }
+            )
+        }
+    }
+
     override fun visitTypeOperatorCall(typeOperatorCall: FirTypeOperatorCall, data: Any?): IrElement {
         return typeOperatorCall.convertWithOffsets { startOffset, endOffset ->
             val irTypeOperand = typeOperatorCall.conversionTypeRef.toIrType(session, declarationStorage)