[FIR] Fix disappeared ILLEGAL_ESCAPE

^KT-59950 Fixed

compiler/testData/diagnostics/tests/StringTemplates.fir.kt

@@ -1,22 +0,0 @@
-fun demo() {
-  val abc = 1
-  val a = ""
-  val asd = 1
-  val bar = 5
-  fun map(f :  () -> Any?) : Int  = 1
-  fun buzz(f :  () -> Any?) : Int  = 1
-  val sdf = 1
-  val foo = 3;
-    "$abc"
-    "$"
-    "$.$.asdf$\t"
-    "asd\$"
-    "asd$a\x"
-    "asd$a$asd$ $<!UNRESOLVED_REFERENCE!>xxx<!>"
-    "fosdfasdo${1 + bar + 100}}sdsdfgdsfsdf"
-    "foo${bar + map {foo}}sdfsdf"
-    "foo${bar + map { "foo" }}sdfsdf"
-    "foo${bar + map {
-      "foo$sdf${ buzz{}}" }}sdfsdf"
-    "a\u \u0 \u00 \u000 \u0000 \u0AaA \u0AAz.length( ) + \u0022b"
-}

compiler/testData/diagnostics/tests/StringTemplates.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun demo() {
   val abc = 1
   val a = ""