Introduce UnsafeVariance annotation

compiler/testData/diagnostics/tests/generics/suppressVarianceConflict.kt

@@ -0,0 +1,17 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE
+
+class A<out T, in E> {
+    fun foo(x: @UnsafeVariance T) {}
+    fun foo(x: @UnsafeVariance T, y: List<@UnsafeVariance T>): @UnsafeVariance E = null!!
+
+    fun bar(): List<@UnsafeVariance E> = null!!
+}
+
+fun foo(x: A<String, Any?>, cs: CharSequence, ls: List<CharSequence>) {
+    val y: A<CharSequence, String> = x
+
+    y.foo(cs)
+    val s: String = y.foo(cs, ls)
+
+    val ls2: List<String> = y.bar()
+}