[ELF] - Optimize .eh_frame section: remove CIE if all FDEs referencing it were removed.

This patch performs little optimization for eh_frame section.
If all FDE`s that referenced CIE are removed then CIE is also removed from output. 
That can happen for example when dropping FDEs that point to dropped sections. Testcase showing that is included.
The same optimization was added to ld about 14 years ago: https://sourceware.org/ml/binutils/2001-12/msg00144.html, gold does not do that it seems.

Differential revision: http://reviews.llvm.org/D15564

llvm-svn: 256638