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/testData/diagnostics/tests/smartCasts/enumEntryMembers_after.kt

@@ -0,0 +1,19 @@
+// !LANGUAGE: +SoundSmartcastForEnumEntries
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// SKIP_TXT
+
+enum class Message(val text: String?) {
+    HELLO("hello"),
+    WORLD("world"),
+    NOTHING(null)
+}
+
+fun printMessages() {
+    Message.HELLO.text!!
+    <!DEBUG_INFO_SMARTCAST!>Message.HELLO.text<!>.length
+
+    Message.NOTHING.text<!UNSAFE_CALL!>.<!>length
+
+    Message.NOTHING.text!!
+    <!DEBUG_INFO_SMARTCAST!>Message.NOTHING.text<!>.length
+}