[Bug c/62194] Add deadfield attribute to ignore initializers for a structure field

[josh at joshtriplett dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62194

--- Comment #1 from Josh Triplett <josh at joshtriplett dot org> ---
An additional corner case: a deadfield, even one that has a default value to
allow reads of it to work rather than erroring out, must be an rvalue, so that
attempts to take the address of it fail.