Do not create a temporary data structure for relocations.

This patch removes `OutRelocations` vector from the InputChunk and
directly consume `Relocations` vector instead. This should make the linker
faster because we don't create a temporary data structure, and it matches
the lld's design principle that we don't create temporary data structures
for object files but instead directly consume mmap'ed data whenever possible.

Differential Revision: https://reviews.llvm.org/D43491

llvm-svn: 325549