psi2ir: Merge constant entries in string template expressions

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

 #KT-28006

compiler/testData/ir/irText/expressions/enumEntryAsReceiver.txt

@@ -19,7 +19,7 @@ FILE fqName:<root> fileName:/enumEntryAsReceiver.kt
         PROPERTY name:value2 visibility:public modality:FINAL flags:val
           FIELD PROPERTY_BACKING_FIELD name:value2 type:kotlin.String visibility:public flags:final
             EXPRESSION_BODY
-              CONST String type=kotlin.String value=OK
+              CONST String type=kotlin.String value="OK"
           FUN DEFAULT_PROPERTY_ACCESSOR name:<get-value2> visibility:public modality:FINAL <> ($this:X.B) returnType:kotlin.String flags:
             correspondingProperty: PROPERTY name:value2 visibility:public modality:FINAL flags:val
             $this: VALUE_PARAMETER name:<this> type:X.B flags: