Wrong StructLayoutAttribute.Size for struct unions with no data fields

[glchapman]

Consider this type:

[<Struct>]
type ABC = A | B | C

F# 9 compiles it to a struct with this attribute (among others):

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Size = 1)]

However, because of the compiler-generated _tag field, the size is actually 4:

> sizeof<ABC>;;                                                        
val it: int = 4

This seems not to cause any problems, but the documentation states "This field must be equal or greater than the total size, in bytes, of the members of the class or structure," which suggests this should be corrected.

[vzarytovskii]

It's probably some flaw in logic around:

fsharp/src/Compiler/CodeGen/IlxGen.fs

Lines 11567 to 11576 in 935b796

	if
	tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
	||
	// Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
	// In that case we generate a dummy field instead
	(cenv.options.workAroundReflectionEmitBugs && not tycon.TyparsNoRange.IsEmpty)
	then
	ILTypeDefLayout.Sequential { Size = None; Pack = None }, ILDefaultPInvokeEncoding.Ansi
	else
	ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi

Should be relatively easy to investigate and test. Should be a good first issue, gonna mark it like this.