psi2ir: Merge constant entries in string template expressions

This is required to handle UCNs properly, see KT-28006.

 #KT-28006

compiler/testData/ir/irText/classes/superCalls.txt

@@ -11,7 +11,7 @@ FILE fqName:<root> fileName:/superCalls.kt
     PROPERTY name:bar visibility:public modality:OPEN flags:val
       FIELD PROPERTY_BACKING_FIELD name:bar type:kotlin.String visibility:public flags:final
         EXPRESSION_BODY
-          CONST String type=kotlin.String value=
+          CONST String type=kotlin.String value=""
       FUN DEFAULT_PROPERTY_ACCESSOR name:<get-bar> visibility:public modality:OPEN <> ($this:Base) returnType:kotlin.String flags:
         correspondingProperty: PROPERTY name:bar visibility:public modality:OPEN flags:val
         $this: VALUE_PARAMETER name:<this> type:Base flags: