Fix bogus smartcast on enum entry members

Previously, enum entries were treated by the data-flow subsystem similar
to other class/singletons. As a consequence, calls like
'Enum.ENTRY.property' had IdentifierInfo of 'property'.

However, specially for enum entries, descriptor of 'property' is one and
the same for all entries. It means that from the data-flow point of
view, 'Enum.ONE.property' and 'Enum.TWO.property' are *one and the same
data-flow values*.

It could obviously lead to some bogus smartcasts, so this commit
introduces separate IdentifierInfo.EnumEntry and uses it to build proper
qualified values.

^KT-20772 Fixed

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/IdentifierInfo.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.resolve.calls.smartcasts
 
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.lexer.KtToken
@@ -77,6 +78,10 @@ interface IdentifierInfo {
         override fun toString() = descriptor.toString()
     }
 
+    data class EnumEntry(val descriptor: ClassDescriptor) : IdentifierInfo {
+        override val kind: DataFlowValue.Kind = STABLE_VALUE
+    }
+
     class Qualified(
         val receiverInfo: IdentifierInfo,
         val selectorInfo: IdentifierInfo,
@@ -229,7 +234,14 @@ private fun getIdForSimpleNameExpression(
             }
         }
 
-        is PackageViewDescriptor, is ClassDescriptor -> IdentifierInfo.PackageOrClass(declarationDescriptor)
+        is ClassDescriptor -> {
+            if (declarationDescriptor.kind == ClassKind.ENUM_ENTRY && languageVersionSettings.supportsFeature(LanguageFeature.SoundSmartcastForEnumEntries))
+                IdentifierInfo.EnumEntry(declarationDescriptor)
+            else
+                IdentifierInfo.PackageOrClass(declarationDescriptor)
+        }
+
+        is PackageViewDescriptor -> IdentifierInfo.PackageOrClass(declarationDescriptor)
 
         else -> IdentifierInfo.NO
     }