[NFC][libunwind][AIX] Add diagnostic push and TODO

clang treats the cast here as an error since 999d4f840777bf8de26d45947192aa0728edc0fb landed
with -Xextra -Werror. We believe this cast to be safe for the reasons noted in comment, but
we should do some further cleanup at some point.

GitOrigin-RevId: 64dc55835a1552dbe74eef79460094014edf659e
1 file changed