Diagnostics: add diagnostic for reporting contract description blocks in old frontend

compiler/testData/diagnostics/testsWithStdLib/contracts/newSyntax/complexContractDescription.kt

@@ -0,0 +1,24 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+import kotlin.contracts.*
+
+fun foo(arg: Any?, num: Int?, block: () -> Unit) contract <!UNSUPPORTED!>[
+    returns() implies (arg is String),
+    returns() implies (num != null),
+    callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+]<!> {
+    require(arg is String)
+    require(num != null)
+    block()
+}
+
+fun bar(arg: Any?, block: () -> Int): Boolean contract <!UNSUPPORTED!>[
+    returns(true) implies (arg != null),
+    callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+]<!> {
+    val num = block()
+    if (arg != null) {
+        return true
+    }
+    return false
+}