"raw" patch adding comdat support to lld elf2

[Rafael Espíndola via llvm-commits]

> How can it be a performance optimization?
>
> Say we have _Z3foov symbol but have never seen _Z3foov group. We are reading
> an object file that contains a group whose name is _Z3foov. Then your code
> will just read that group regardless of the presence of the symbol having
> the same name, no?

The optimization is just saving space by not having another
independent set keyed on symbol names. With this patch the comdat
signature foo and the symbol foo only use a map to a
PointIntergerPair.

A new version is at http://reviews.llvm.org/D13598.

Cheers,
Rafael