replace 'trait' keyword with 'interface' in user-visible messages and code generation tools

compiler/testData/diagnostics/tests/subtyping/kt2069.txt

@@ -18,7 +18,7 @@ package kt2069 {
         }
     }
 
-    internal trait T1 {
+    internal interface T1 {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         internal open fun foo(): kotlin.Unit
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int

compiler/testData/diagnostics/tests/subtyping/kt3159.txt

@@ -9,7 +9,7 @@ internal final class Sub : Super {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
-internal trait Super {
+internal interface Super {
     internal abstract var v: kotlin.CharSequence
     internal abstract val v2: kotlin.CharSequence
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean