[ELF] - Dedupliсate FDEs when sections are ICFed.

When LLD do ICF for 2 identical sections it leaves 2 duplicate entries in .eh_frame
pointing to the same address. After that it fixes .eh_frame_header's header,
so that it says it contains single FDE, though section itself contains 2 
(it contains garbage data at tail).

As a result excessive entries in .eh_frame and excessive dummy data in .eh_frame_header
emited to output. Patch fixes that. This is PR34518.

Differential revision: https://reviews.llvm.org/D38998

llvm-svn: 316648