Simplify the handling of aliases in the gold plugin.

The complicated situation is when we have to keep an alias but drop a GV
that is part of the aliasee.

We used to clone the dropped GV and make the clone internal. This is wasteful
as we know the original will be dropped.

With this patch what is done instead is set the linkage of the original to
internal and replace all uses (but the one in the alias) with a new
declaration that takes the name of the old GV. This saves us from having
to copy the body.

llvm-svn: 223863