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/kt28006.kt

@@ -0,0 +1,13 @@
+val test1 = "\uD83E\uDD17"
+val test2 = "\uD83E\uDD17\uD83E\uDD17"
+
+const val testConst1 = "\uD83E\uDD17"
+const val testConst2 = "\uD83E\uDD17\uD83E\uDD17"
+const val testConst3 = "\uD83E\uDD17$testConst2"
+const val testConst4 = "$testConst2$testConst2"
+
+fun test1(x: Int) = "\uD83E\uDD17$x"
+
+fun test2(x: Int) = "$x\uD83E\uDD17"
+
+fun test3(x: Int) = "$x\uD83E\uDD17$x"