Rename DynamicReloc::getAddend() to computeAddend(). NFC

Summary:
Before the name of the function sounded like it was just a getter for the
private class member Addend. However, it actually calculates the final
value for the r_addend field in Elf_Rela that is used when writing the
.rela.dyn section. I also added a comment to the UseSymVA member to
explain how it interacts with computeAddend().

Differential Revision: https://reviews.llvm.org/D43161

llvm-svn: 325485