cmd/cgo: fix mangling of enum and union types

cmd/cgo: fix mangling of enum and union types

Consider this test package:

    package p

    // enum E { E0 };
    // union U { long x; };
    // void f(enum E e, union U* up) {}
    import "C"

    func f() {
    	C.f(C.enum_E(C.E0), (*C.union_U)(nil))
    }

In Go 1.14, cgo translated this to (omitting irrelevant details):

    type _Ctype_union_U [8]byte

    func f() {
    	_Cfunc_f(uint32(_Ciconst_E0), (*[8]byte)(nil))
    }

    func _Cfunc_f(p0 uint32, p1 *[8]byte) (r1 _Ctype_void) { ... }

Notably, _Ctype_union_U was declared as a defined type, but uses were
being rewritten into uses of the underlying type, which matched how
_Cfunc_f was declared.

After CL 230037, cgo started consistently rewriting "C.foo" type
expressions as "_Ctype_foo", which caused it to start emitting:

    type _Ctype_enum_E uint32
    type _Ctype_union_U [8]byte

    func f() {
    	_Cfunc_f(_Ctype_enum_E(_Ciconst_E0), (*_Ctype_union_U)(nil))
    }

    // _Cfunc_f unchanged

Of course, this fails to type-check because _Ctype_enum_E and
_Ctype_union_U are defined types.

This CL changes cgo to emit:

    type _Ctype_enum_E = uint32
    type _Ctype_union_U = [8]byte

    // f unchanged since CL 230037
    // _Cfunc_f still unchanged

It would probably be better to fix this in (*typeConv).loadType so
that cgo generated code uses the _Ctype_foo aliases too. But as it
wouldn't have any effect on actual compilation, it's not worth the
risk of touching it at this point in the release cycle.

Updates #39537.
Fixes #40494.

Change-Id: I88269660b40aeda80a9a9433777601a781b48ac0
Reviewed-on: https://go-review.googlesource.com/c/go/+/246057
Reviewed-by: Ian Lance Taylor <iant@golang.org>