Add CopyRelSection instances to BSS in the regular way.

Previously, space in a BSS section for copy relocations are reserved
in a special way. We directly manipulated size of the BSS section.
r294577 changed the way of doing it. Now, we create an instance of
CopyRelSection (which is a synthetic input section) for each copy
relocation.

This patch removes the remains of the old way and add CopyRelSections
to BSS sections using `addSections` function, which is the usual
way to add an input section to an output section.

llvm-svn: 295278