No FSharpSymbolUse for single case union type

[vasily-kirichenko]

type DU = Case

FSharpCheckFileResult.GetAllUsesOfAllSymbolsInFile() return a FSharpSymbolUse for DU and for the implicit module, but not for Case. As a result, VFPT cannot colorize Case as pattern.
See fsprojects-archive/zzarchive-VisualFSharpPowerTools#934

[dsyme]

I think adding

                writeFakeUnionCtorsToSink [ unionCase ]

here:

            | TyconCoreAbbrevThatIsReallyAUnion (hasMeasureAttr,envinner,id) (unionCaseName,_) ->
                structLayoutAttributeCheck(false)
                noAllowNullLiteralAttributeCheck()
                TcRecdUnionAndEnumDeclarations.CheckUnionCaseName  cenv unionCaseName.idText unionCaseName.idRange
                let unionCase = NewUnionCase unionCaseName unionCaseName.idText [] thisTy [] XmlDoc.Empty tycon.Accessibility
                writeFakeUnionCtorsToSink [ unionCase ]
                MakeUnionRepr [ unionCase ], None, NoSafeInitInfo

in tc.fs will fix this.

@vasily-kirichenko - do you think you could you take a look and submit a PR with tests? If not let's try to recruit someone who'd like to contribute to this repo, since I think this is low-hanging fruit bug.

Thanks
don

[vasily-kirichenko]

@dsyme done #308
thanks!