add compile error for copying an async frame

[andrewrk]

export fn a() void {
    const f = async func();
    var f2 = f;
    resume f2;
}
fn func() void {
    suspend;
}

Expected output: error: cannot copy @Frame(func)

Actual output: builds successfully

This cannot work because local variables can, for example, be pointers to each other. Copying the frame would break this.