Fix warnings in stdlib/compiler/plugins/test code

compiler/frontend/src/org/jetbrains/kotlin/checkers/diagnostics/ActualDiagnostic.kt

@@ -17,7 +17,7 @@ class ActualDiagnostic constructor(val diagnostic: Diagnostic, override val plat
         TextDiagnostic.InferenceCompatibility.OLD
 
     override val name: String
-        get() = diagnostic.factory.name!!
+        get() = diagnostic.factory.name
 
     val file: PsiFile
         get() = diagnostic.psiFile

compiler/frontend/src/org/jetbrains/kotlin/checkers/utils/DebugInfoUtil.kt

@@ -205,7 +205,7 @@ object DebugInfoUtil {
     }
 
     abstract class DebugInfoReporter {
-        fun preProcessReference(expression: KtReferenceExpression) {
+        fun preProcessReference(@Suppress("UNUSED_PARAMETER") expression: KtReferenceExpression) {
             // do nothing
         }
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/DiagnosticFactoryWithPsiElement.java

@@ -18,6 +18,7 @@ package org.jetbrains.kotlin.diagnostics;
 
 import com.intellij.openapi.util.TextRange;
 import com.intellij.psi.PsiElement;
+import kotlin.DeprecationLevel;
 
 import java.util.List;
 
@@ -43,6 +44,7 @@ public abstract class DiagnosticFactoryWithPsiElement<E extends PsiElement, D ex
 
     @SuppressWarnings("MethodOverloadsMethodOfSuperclass")
     @Deprecated
+    @kotlin.Deprecated(message = "Use `cast` from the superclass.", level = DeprecationLevel.HIDDEN)
     // ABI-compatibility only (used in Android plugin)
     public D cast(Diagnostic d) {
         return super.cast(d);

compiler/frontend/src/org/jetbrains/kotlin/resolve/diagnostics/KotlinSuppressCache.kt

@@ -164,7 +164,7 @@ abstract class KotlinSuppressCache {
 
     companion object {
         private fun getDiagnosticSuppressKey(diagnostic: Diagnostic): String =
-            diagnostic.factory.name!!.toLowerCase()
+            diagnostic.factory.name.toLowerCase()
 
         private fun isSuppressedByStrings(key: String, strings: Set<String>, severity: Severity): Boolean =
             severity == Severity.WARNING && "warnings" in strings || key in strings