Support suspend functions as superinterfaces

Forbid mixing suspend and non-suspend functional supertypes.
Since JVM BE generates suspend functional types as non-suspend ones
with SuspendFunction marker interface, there is not way to distinguish
non-suspend functional type from suspend one if they are mixed.
 #KT-18707 Fixed

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -205,6 +205,8 @@ enum class LanguageFeature(
     DefinitelyNotNullTypeParameters(KOTLIN_1_6),
     ProhibitUsingNullableTypeParameterAgainstNotNullAnnotated(KOTLIN_1_6, kind = BUG_FIX),
 
+    SuspendFunctionAsSupertype(KOTLIN_1_6),
+
     // Temporarily disabled, see KT-27084/KT-22379
     SoundSmartcastFromLoopConditionForLoopAssignedVariables(sinceVersion = null, kind = BUG_FIX),