[ELF] Split EhInputSection::pieces into cies and fdes

This simplifies code, removes a read32 (for id==0 check), and makes it feasible
to combine some operations in EhInputSection::split and EhFrameSection::addRecords.

Mostly NFC, but fixes "Relocation not in any piece" assertion failure in an
erroneous case when a relocation offset precedes all CIE/FDE pices.