This feature is enabled by default since 1.4, which is the earliest language version supported by Kotlin at this moment.
Had to edit some bytecodeText tests to account for the fact that JVM_IR no longer generates explicit initializations for ConstantValue fields, but NoConstantValueAttributeForNonConstVals is not the default yet.
The newly added pass folds the set of constant functions of the current backend, plus IrBuiltIns.