Investigate why IsMethod and IsProperty don't act as expected for generated methods/properties

[abonie]

Follow up to #17301 - there you can see the discrepancy described in this comment #17301 (comment)

This discrepancy could be a source of more undiscovered bugs similar to 17301

Repro steps

Inspect a generated code like IsUnionCaseTester property, or Equals method by enumerating declarations from assembly contents, like in the snippet from the comment linked above:

    for impl_file in projectResults.AssemblyContents.ImplementationFiles do
        for file_decl in impl_file.Declarations do
            match file_decl with
            | FSharpImplementationFileDeclaration.Entity (ent, ent_decls) ->
                for ent_decl in ent_decls do
                    match ent_decl with
                    | FSharpImplementationFileDeclaration.MemberOrFunctionOrValue (mfv, args, body) ->
                        if mfv.CompiledName.StartsWith("get_Is") then
                            printfn $"""
--- when enumerating declarations ---
{mfv.CompiledName}:
    IsProperty = {mfv.IsProperty}
    IsMethod = {mfv.IsMethod}
    IsFunction = {mfv.IsFunction}
    IsPropertyGetterMethod = {mfv.IsPropertyGetterMethod}
    IsValue = {mfv.IsValue}"""
                    | _ -> ()
            | _ -> ()

vs

Inspect uses of these generated properties/methods in user code with GetSymbolUseAtLocation

Expected behavior

IsUnionCaseTester property always has IsProperty = true and Equals always has IsMethod = true

Actual behavior

When enumerating declarations in generated code, these generated properties/methods are not marked as such.