Effects: add diagnostic tests on functions from stdlib

==========
Introduction of EffectSystem: 18/18

compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/check.kt

@@ -0,0 +1,28 @@
+// !LANGUAGE: +ReturnsEffect
+// !DIAGNOSTICS: -INVISIBLE_REFERENCE -INVISIBLE_MEMBER
+
+fun testCheckSmartcast(x: Any?) {
+    check(x is String)
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length
+}
+
+fun testCheckUnreachableCode() {
+    check(false)
+    // Can't be reported without notion of 'iff'
+    println("Can't get here!")
+}
+
+fun testCheckWithMessage(x: Any?) {
+    check(x is String) { "x is not String!" }
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length
+}
+
+fun testCheckWithFailingMessage(x: Any?) {
+    check(x is String) { throw kotlin.IllegalStateException("What a strange idea") }
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length
+}
+
+fun tesCheckNotNullWithMessage(x: Int?) {
+    checkNotNull(x) { "x is null!"}
+    <!DEBUG_INFO_SMARTCAST!>x<!>.inc()
+}