cmd/cgo: fix check for conversion of ptr to struct field

According to the documentation "When passing a pointer to a field in a
struct, the Go memory in question is the memory occupied by the field,
not the entire struct.". checkAddr states that this should also work
with type conversions, which is implemented in isType. However,
ast.StarExpr must be enclosed in ast.ParenExpr according to the go spec
(see example below), which is not considered in the checks.

Example:
    // struct Si { int i; int *p; }; void f(struct I *x) {}
    import "C"
    type S {
        p *int
        i C.struct_Si
    }
    func main() {
        v := &S{new(int)}
        C.f((*C.struct_I)(&v.i)) // <- panic
    }

This example will cause cgo to emit a cgoCheck that checks the whole
struct S instead of just S.i causing the panic "cgo argument has Go
pointer to Go pointer".

This patch fixes this situation by adding support for ast.ParenExpr to
isType and adds a test, that fails without the fix.

Fixes #32970.

Change-Id: I15ea28c98f839e9fa708859ed107a2e5f1483133
Reviewed-on: https://go-review.googlesource.com/c/go/+/185098


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>