[FIR] Create string interpolating call even for single argument

Before this commit, questionable optimization existed which
unwrapped string interpolating call with single argument to this argument.
However, this led to source element loss and the necessity of sub-hacks.
In this commit we dropped this optimization (anyway user can remove
this single-expression string template in code if needed) to keep
source elements intact.

compiler/testData/codegen/bytecodeText/stringOperations/kt42457_old.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KOTLIN_CONFIGURATION_FLAGS: STRING_CONCAT=indy-with-constants
 // JVM_TARGET: 9
 // FILE: JavaClass.java

compiler/testData/codegen/bytecodeText/stringOperations/singleConcat.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun f(s: String) = "$s"
 
 fun g(s: String?) = "$s"