[patch] Add support for putting constructors and destructos in explicit comdats

[Rafael Espíndola]

Hi Reid,

The attached patch is rebased on top of trunk, implements the rules
described in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62306#c6 and
I think has all your previous suggestions.

I have sent an email to cxx-abi-dev documenting the above rules, but
got no replies so far.

Cheers,
Rafael