Introduce LanguageFeature.State, drop coroutines-related pseudofeatures

Previously there were three LanguageFeature instances -- Coroutines,
DoNotWarnOnCoroutines and ErrorOnCoroutines -- which were handled very
awkwardly in the compiler and in the IDE to basically support a language
feature with a more complex state: not just enabled/disabled, but also
enabled with warning and enabled with error. Introduce a new enum
LanguageFeature.State for this and allow LanguageVersionSettings to get
the state of any language feature with 'getFeatureSupport'.

One noticeable drawback of this approach is that looking at the API, one
may assume that any language feature can be in one of the four states
(enabled, warning, error, disabled). This is not true however; there's
only one language feature at the moment (coroutines) for which these
intermediate states (warning, error) are handled in any way. This may be
refactored further by abstracting the logic that checks the language
feature availability so that it would work exactly the same for any
feature.

Another issue is that the difference among ENABLED_WITH_ERROR and
DISABLED is not clear. They are left as separate states because at the
moment, different diagnostics are reported in these two cases and
quick-fixes in IDE rely on that

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

@@ -1,16 +1,16 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
-// !LANGUAGE: +ErrorOnCoroutines
+// !LANGUAGE: -Coroutines
 
-<!EXPERIMENTAL_FEATURE_ERROR!>suspend<!> fun suspendHere(): String = "OK"
+<!UNSUPPORTED_FEATURE!>suspend<!> fun suspendHere(): String = "OK"
 
-fun builder(c: <!EXPERIMENTAL_FEATURE_ERROR!>suspend<!> () -> Unit) {
+fun builder(c: <!UNSUPPORTED_FEATURE!>suspend<!> () -> Unit) {
 
 }
 
 fun box(): String {
     var result = ""
 
-    <!EXPERIMENTAL_FEATURE_ERROR!>builder<!> {
+    <!UNSUPPORTED_FEATURE!>builder<!> {
         suspendHere()
     }
 

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

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

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

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