Names containing only '_' are deprecated (declarations, parameters, import aliases, labels, but not backquoted names)

compiler/testData/diagnostics/tests/Underscore.kt

@@ -0,0 +1,16 @@
+// !DIAGNOSTICS: -DEPRECATED_SYMBOL_WITH_MESSAGE
+
+<!UNDESCORE_IS_DEPRECATED!>import kotlin.Deprecated as ___<!>
+
+@___("") data class Pair(val x: Int, val y: Int)
+
+class <!UNDESCORE_IS_DEPRECATED!>_<!><<!UNDESCORE_IS_DEPRECATED!>________<!>>
+val <!UNDESCORE_IS_DEPRECATED!>______<!> = _<Int>()
+
+fun <!UNDESCORE_IS_DEPRECATED!>__<!>(<!UNDESCORE_IS_DEPRECATED!>___<!>: Int, y: _<Int>?): Int {
+    val (x, <!UNDESCORE_IS_DEPRECATED!>__________<!>) = Pair(___ - 1, 42)
+    val <!UNDESCORE_IS_DEPRECATED!>____<!> = x
+    // in backquotes: allowed
+    val `_` = __________
+    <!UNDESCORE_IS_DEPRECATED!>__<!>@ return if (y != null) __(____, y) else __(`_`, ______)
+}