ELF: Create dynamic symbols for symbol aliases of copy relocations.

For each copy relocation that we create, look through the DSO's symbol table
for aliases and create a dynamic symbol for each one. This causes the copy
relocation to correctly interpose any aliases.

Copy relocations are relatively uncommon (on my machine, 56% of binaries in
/usr/bin have no copy relocations probably due to being PIEs, 97% of them
have <10, and the binary with the largest number of them has 97) so it's
probably fine to do this in a relatively inefficient way.

Differential Revision: http://reviews.llvm.org/D18731

llvm-svn: 265354