ELF: Reduce the size of InputSectionBase by two words. NFCI.

- The Assigned bit was previously taking a word on its own. Move
  it into the bit fields in SectionBase.
- NumRelocations and AreRelocsRela were previously also taking up
  a word despite only using half of it. Move them into the alignment gap
  after SectionBase's fields.

Differential Revision: https://reviews.llvm.org/D59044

llvm-svn: 355622