Introduce keys to control applicability of coroutines

By default now we produce warnings on coroutines

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

@@ -0,0 +1,18 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// !LANGUAGE: +ErrorOnCoroutines
+
+<!EXPERIMENTAL_FEATURE_ERROR!>suspend<!> fun suspendHere(): String = "OK"
+
+fun builder(c: suspend () -> Unit) {
+
+}
+
+fun box(): String {
+    var result = ""
+
+    <!EXPERIMENTAL_FEATURE_ERROR!>builder<!> {
+        suspendHere()
+    }
+
+    return result
+}

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

@@ -0,0 +1,5 @@
+package
+
+public fun box(): kotlin.String
+public fun builder(/*0*/ c: suspend () -> kotlin.Unit): kotlin.Unit
+public suspend fun suspendHere(): kotlin.String

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

@@ -0,0 +1,18 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// !LANGUAGE: +WarnOnCoroutines
+
+<!EXPERIMENTAL_FEATURE_WARNING!>suspend<!> fun suspendHere(): String = "OK"
+
+fun builder(c: suspend () -> Unit) {
+
+}
+
+fun box(): String {
+    var result = ""
+
+    <!EXPERIMENTAL_FEATURE_WARNING!>builder<!> {
+        suspendHere()
+    }
+
+    return result
+}

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

@@ -0,0 +1,5 @@
+package
+
+public fun box(): kotlin.String
+public fun builder(/*0*/ c: suspend () -> kotlin.Unit): kotlin.Unit
+public suspend fun suspendHere(): kotlin.String