Completely rewrite ELFObjectWriter::RecordRelocation.

I started trying to fix a small issue, but this code has seen a small fix too
many.

The old code was fairly convoluted. Some of the issues it had:

* It failed to check if a symbol difference was in the some section when
  converting a relocation to pcrel.
* It failed to check if the relocation was already pcrel.
* The pcrel value computation was wrong in some cases (relocation-pc.s)
* It was missing quiet a few cases where it should not convert symbol
  relocations to section relocations, leaving the backends to patch it up.
* It would not propagate the fact that it had changed a relocation to pcrel,
  requiring a quiet nasty work around in ARM.
* It was missing comments.

llvm-svn: 205076