Make it explicit that we are writing addends to target sections if REL.

Relocations.cpp is still head-scratching. Even though relocations are
processed by multiple functions, the functions are effectively one
gigantic function with lots of local and global shared states, because
they are really tightly coupled. It is really hard to predict whether
a change to a function will or will not affect other functions behaviors.

What I'm trying to do is to rewrite the code without breaking the
existing tests so that the code can tolerate a more aggressive
refactoring (i.e. splitting it to logically separated steps).

llvm-svn: 315673