[ELF] EhFrameSection: postpone FDE liveness check to finalizeSections

EhFrameSection::addSection checks liveness of FDE early. This makes it
infeasible to move combineEhSections() before ICF.

Postpone the check to EhFrameSection::finalizeContents(). This is what
ARMExidxSyntheticSection does and it will make a subsequent patch D66717
simpler.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D66727

llvm-svn: 369890