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

compiler/testData/diagnostics/tests/inline/nonVirtualMembersWithInline.txt

@@ -21,7 +21,7 @@ internal abstract class A {
     kotlin.inline() internal abstract fun wrong6(): kotlin.Unit
 }
 
-internal trait B {
+internal interface B {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     kotlin.inline() private final fun good1(): kotlin.Unit
     kotlin.inline() public final fun good2(): kotlin.Unit