Add checks that coroutines are available at the given language level

compiler/testData/diagnostics/tests/coroutines/unsupported.kt

@@ -0,0 +1,24 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// !LANGUAGE: -Coroutines
+
+class Controller {
+    <!UNSUPPORTED_FEATURE!>suspend<!> fun suspendHere(x: Continuation<String>) {
+        x.resume("OK")
+    }
+
+    <!UNSUPPORTED_FEATURE!>operator<!> fun handleResult(x: String, y: Continuation<Nothing>) {}
+}
+
+fun builder(<!UNSUPPORTED_FEATURE!>coroutine<!> c: Controller.() -> Continuation<Unit>) {
+    c(Controller()).resume(Unit)
+}
+
+fun box(): String {
+    var result = ""
+
+    <!UNSUPPORTED_FEATURE!>builder<!> {
+        suspendHere()
+    }
+
+    return result
+}

compiler/testData/diagnostics/tests/coroutines/unsupported.txt

@@ -0,0 +1,13 @@
+package
+
+public fun box(): kotlin.String
+public fun builder(/*0*/ coroutine c: Controller.() -> kotlin.coroutines.Continuation<kotlin.Unit>): kotlin.Unit
+
+public final class Controller {
+    public constructor Controller()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final operator fun handleResult(/*0*/ x: kotlin.String, /*1*/ y: kotlin.coroutines.Continuation<kotlin.Nothing>): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final suspend fun suspendHere(/*0*/ x: kotlin.coroutines.Continuation<kotlin.String>): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}