FIR: report ENUM_ENTRY_AS_TYPE and IS_ENUM_TYPE

These two diagnostics are similar: both are reported on type references
to enum entries. But `IS_ENUM_TYPE` is reported if the type ref is an
operand of `is` operator. To pass along this contextual information, a
boolean is added to FirSpecificTypeResolverTransformer.

compiler/testData/diagnostics/tests/enum/isEnumEntry.fir.kt

@@ -3,4 +3,4 @@ enum class MyEnum {
     SECOND
 }
 
-fun foo(me: MyEnum): Boolean = me is <!UNRESOLVED_REFERENCE!>MyEnum.FIRST<!>
+fun foo(me: MyEnum): Boolean = me is <!IS_ENUM_ENTRY!>MyEnum.FIRST<!>