Merge two deprecation diagnostics into one DEPRECATION

This also fixes weird suppressions in user code: instead of
«@Suppress("DEPRECATED_SYMBOL_WITH_MESSAGE")» you should now use
«@Suppress("DEPRECATION")»

compiler/testData/diagnostics/tests/Underscore.kt

@@ -1,4 +1,4 @@
-// !DIAGNOSTICS: -DEPRECATED_SYMBOL_WITH_MESSAGE
+// !DIAGNOSTICS: -DEPRECATION
 
 <!UNDERSCORE_IS_DEPRECATED!>import kotlin.Deprecated as ___<!>
 
@@ -13,4 +13,4 @@ fun <!UNDERSCORE_IS_DEPRECATED!>__<!>(<!UNDERSCORE_IS_DEPRECATED!>___<!>: Int, y
     // in backquotes: allowed
     val `_` = __________
     <!UNDERSCORE_IS_DEPRECATED!>__<!>@ return if (y != null) __(____, y) else __(`_`, ______)
-}
+}