Add test for UNUSED_VALUE on when subject variable

compiler/testData/diagnostics/tests/when/withSubjectVariable/unusedWhenSubjectVariable.kt

@@ -0,0 +1,8 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun foo(): Any = 42
+
+fun test() {
+    when (val <!UNUSED_VARIABLE!>x<!> = foo()) {
+    }
+}

compiler/testData/diagnostics/tests/when/withSubjectVariable/unusedWhenSubjectVariable.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(): kotlin.Any
+public fun test(): kotlin.Unit

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -22501,6 +22501,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/withSubjectVariable/unsupportedVariableDeclarationsInWhenSubject.kt");
                     doTest(fileName);
                 }
+
+                @TestMetadata("unusedWhenSubjectVariable.kt")
+                public void testUnusedWhenSubjectVariable() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/withSubjectVariable/unusedWhenSubjectVariable.kt");
+                    doTest(fileName);
+                }
             }
         }
     }

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -22501,6 +22501,12 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/withSubjectVariable/unsupportedVariableDeclarationsInWhenSubject.kt");
                     doTest(fileName);
                 }
+
+                @TestMetadata("unusedWhenSubjectVariable.kt")
+                public void testUnusedWhenSubjectVariable() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/withSubjectVariable/unusedWhenSubjectVariable.kt");
+                    doTest(fileName);
+                }
             }
         }
     }