Invert LanguageFeature responsible for warning on coroutines

The problem was that LanguageVersionSettingsImpl.DEFAULT did not have
"WarnOnCoroutines" as a feature and so it was manually added to the settings,
but only in two places: in the compiler and in the IDE

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

@@ -1,5 +1,5 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
-// !LANGUAGE: +WarnOnCoroutines
+// !LANGUAGE: -DoNotWarnOnCoroutines
 
 <!EXPERIMENTAL_FEATURE_WARNING!>suspend<!> fun suspendHere(): String = "OK"
 

compiler/testData/diagnostics/testsWithStdLib/coroutines/suspendCoroutineUnavailableWithNewAPI.kt

@@ -1,4 +1,5 @@
 // !API_VERSION: 1.1
+// !LANGUAGE: +DoNotWarnOnCoroutines
 // SKIP_TXT
 
 import kotlin.coroutines.experimental.*