gccrs: cleanup our enum type layout to be closer to rustc

[philberty]

This changes our enum type layout so for example:

  enum Foo {
      A,
      B,
      C(char),
      D { x: i32, y: i32 },
  }

Used to get layed out like this in gccrs:

  union {
    struct A { int RUST$ENUM$DISR; };
    struct B { int RUST$ENUM$DISR; };
    struct C { int RUST$ENUM$DISR; char __0; };
    struct D { int RUST$ENUM$DISR; i64 x; i64 y; };
  }

This has some issues notably with the constexpr because this is just a giant union it means its not simple to constify what enum variant we are looking at because the discriminant is a mess.

This now gets layed out as:

  struct {
     int RUST$ENUM$DISR;
     union {
         struct A { };
         struct B { };
         struct C { char __0; };
         struct D { i64 x; i64 y; };
     } payload;
  }

This layout is much cleaner and allows for our constexpr to work properly.

gcc/rust/ChangeLog:

* backend/rust-compile-expr.cc (CompileExpr::visit): new layout
* backend/rust-compile-pattern.cc (CompilePatternCheckExpr::visit): likewise
(CompilePatternBindings::visit): likewise
* backend/rust-compile-resolve-path.cc: likewise
* backend/rust-compile-type.cc (TyTyResolveCompile::visit): implement new layout
* rust-gcc.cc (constructor_expression): get rid of useless assert

[philberty]

our gdb output looks much better with this too.

[P-E-P]

I'm not familiar enough with all details here to provide an appropriate review but it looks great and I couldn't find anything to say.

I wonder if this would fix one failing test in #3299

[P-E-P]

I'm not sure why we need that variable ? Could we use folded_discrim_expr directly or rename it ?

[philberty]

good find will fix