[DWARF] Refactor RelocVisitor and fix computation of SHT_RELA-typed relocation entries

Summary: This is the lld-side change of D57939

Reviewers: echristo, dblaikie, ruiu, espindola

Reviewed By: echristo, ruiu

Subscribers: jdoerfert, emaste, arichardson, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57940

llvm-svn: 356730