Instead of putting the magic in the AST, generate regular accessor
methods. This avoids a number of special cases in the compiler, and
avoids missing any of them.
The resulting union accesses are somewhat clunkier to use, but the
compiler implementation has far less coupling between the CGo
implementation and the IR generator.
Enum types are implemented as named types (with possible accompanying
typedefs as type aliases). The constants inside the enums are treated as
Go constants like in the Go toolchain.
These types (called elaborated types in C) are used as part of linked
lists, among others.
This is part an extra feature (to be compatible with CGo C.struct_
types) and part a bugfix: linked lists would result in endless recursion
leading to a stack overflow.
Unions are somewhat hard to implement in Go because they are not a
native type. But it is actually possible with some compiler magic.
This commit inserts a special "C union" field at the start of a struct
to indicate that it is a union. As such a field cannot be written
directly in Go, this is a useful to distinguish structs and unions.
