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/when/ExhaustiveEnumIs.fir.kt

@@ -13,8 +13,8 @@ enum class MyEnum {
 
 fun foo(x: MyEnum): Int {
     return <!NO_ELSE_IN_WHEN!>when<!> (x) {
-        is <!UNRESOLVED_REFERENCE!>MyEnum.A<!> -> 1
-        is <!UNRESOLVED_REFERENCE!>MyEnum.B<!> -> 2
-        is <!UNRESOLVED_REFERENCE!>MyEnum.C<!> -> 3
+        is <!IS_ENUM_ENTRY!>MyEnum.A<!> -> 1
+        is <!IS_ENUM_ENTRY!>MyEnum.B<!> -> 2
+        is <!IS_ENUM_ENTRY!>MyEnum.C<!> -> 3
     }
 }

compiler/testData/diagnostics/tests/when/ExhaustiveEnumMixed.fir.kt

@@ -15,7 +15,7 @@ enum class MyEnum {
 fun foo(x: MyEnum): Int {
     return <!NO_ELSE_IN_WHEN!>when<!> (x) {
         MyEnum.A -> 1
-        is <!UNRESOLVED_REFERENCE!>MyEnum.B<!> -> 2
-        is <!UNRESOLVED_REFERENCE!>MyEnum.C<!> -> 3
+        is <!IS_ENUM_ENTRY!>MyEnum.B<!> -> 2
+        is <!IS_ENUM_ENTRY!>MyEnum.C<!> -> 3
     }
 }