[<Struct>] DUs - eliminate dummy .ctor args, fix bug > 49 cases, simplify IL

[T-Gro]

Fixes: #5342
Fixes: #9767

Before:
https://sharplab.io/#v2:DYLgZgzgPgsAUAbQDwDkD2BhNBbADgQwCcBLCNAOwG50BRARwFd9hiAXAT0oGVXCGBjVgD4AuvA64ApgAIAKhAjSAvNOkBBaVGkAhTdIx6AInpp6AYnoDiegBJ6AknoBSegNJ6AMnoCyelPHggA=

Have a look at the generated constructors, artificial arguments were added to tell them apart.
This lead to unexpected runtime crashes when constructing a case from a DU with > 49 cases.

After:

• Nullary cases created plain .ctor setting the _tag field (just like new StructType(_tag = 13))
• Non-nullary cases created using "maker method", which:
  • initializes the struct to 0 ( I am open to suggestions about cases where we could in theory avoid it?)
  • assigns the constant _tag field for this case
  • assigns fields needed by the case

The public-API - mainly properties for creationg, "IsXXX" properties etc. all remain the same, as does the struct layout in memory (at least for this PR).

[vzarytovskii]

Important question - do we merge it, so it gets to 8, or is it too risky?

[T-Gro]

For me the risk outweighs the existing error System.InvalidProgramException: Common Language Runtime detected an invalid program. which this fixes.

A downside is that we are not dogfooding large struct DUs ourselves that much, only a few small ones in FSharp.Core.

[psfinaki]

LGTM, I guess this can go?