[ELF] Refactor -z combreloc

* `RelocationBaseSection::addReloc` increases `numRelativeRelocs`, which
  duplicates the work done by RelocationSection<ELFT>::writeTo.
* --pack-dyn-relocs=android has inappropropriate DT_RELACOUNT.
  AndroidPackedRelocationSection does not necessarily place relative relocations
  in the front and DT_RELACOUNT might cause semantics error (though our
  implementation doesn't and Android bionic doesn't use DT_RELACOUNT anyway.)

Move `llvm::partition` to a new function `partitionRels` and compute
`numRelativeRelocs` there. Now `RelocationBaseSection::addReloc` is trivial and
can be moved to the header to enable inlining.

The rest of DynamicReloc and `-z combreloc` handling is moved to the
non-template `RelocationBaseSection::computeRels` to decrease code size. My
x86-64 lld executable is 44+KiB smaller.

While here, rename `sort` to `combreloc`.