Fix 452 - FSharpField.IsMutable = true for BCL enum cases

Author: dsyme

src/fsharp/vs/Symbols.fs

@@ -656,7 +656,7 @@ and FSharpField(cenv, d: FSharpFieldData)  =
         if isUnresolved() then false else 
         match d.TryRecdField with 
         | Choice1Of2 r -> r.IsMutable
-        | Choice2Of2 f -> not f.IsInitOnly
+        | Choice2Of2 f -> not f.IsInitOnly && f.LiteralValue.IsNone
 
     member __.IsLiteral = 
         if isUnresolved() then false else 

tests/service/ProjectAnalysisTests.fs

@@ -2907,7 +2907,7 @@ let ``Test Project19 all symbols`` () =
             ("DayOfWeek", "DayOfWeek", "file1", ((10, 15), (10, 24)), [],
              ["enum"; "valuetype"]);
             ("System", "System", "file1", ((10, 8), (10, 14)), [], ["namespace"]);
-            ("field Monday", "Monday", "file1", ((10, 8), (10, 31)), [], ["field"; "mutable"; "static"; "1"]);
+            ("field Monday", "Monday", "file1", ((10, 8), (10, 31)), [], ["field"; "static"; "1"]);
             ("val s", "s", "file1", ((10, 4), (10, 5)), ["defn"], ["val"]);
             ("Impl", "Impl", "file1", ((2, 7), (2, 11)), ["defn"], ["module"])|]