[K2] Disappeared UNSUPPORTED_FEATURE (2)

Added a check for the enabled context receivers feature flag for declaration.

^KT-59913

compiler/testData/diagnostics/testsWithJvmBackend/contextReceivers/kt59590WithContextReceiver.fir.diag.txt

@@ -2,6 +2,8 @@
     context(O) val INSTANCE: O? defined in O
     val INSTANCE: O defined in O
 
+/kt59590WithContextReceiver.fir.kt:5:5: error: The feature "context receivers" is experimental and should be enabled explicitly
+
 /kt59590WithContextReceiver.fir.kt:5:5: error: Platform declaration clash: The following declarations have the same JVM signature (INSTANCELO;):
     context(O) val INSTANCE: O? defined in O
     val INSTANCE: O defined in O

compiler/testData/diagnostics/testsWithJvmBackend/contextReceivers/kt59590WithContextReceiver.fir.kt

@@ -2,7 +2,7 @@
 // !RENDER_ALL_DIAGNOSTICS_FULL_TEXT
 
 <!CONFLICTING_JVM_DECLARATIONS!>object O {
-    <!CONFLICTING_JVM_DECLARATIONS!>context(O)
+    <!CONFLICTING_JVM_DECLARATIONS!><!UNSUPPORTED_FEATURE!>context(O)<!>
     @JvmField
     val INSTANCE: O? = null<!>
-}<!>
+}<!>