[ELF] - Fix segfault when processing .eh_frame.

Its a PR34648 which was a segfault that happened because
we stored pointers to elements in DenseMap. 
When DenseMap grows such pointers are invalidated.
Solution implemented is to keep elements by pointer
and not by value.

Differential revision: https://reviews.llvm.org/D38034

llvm-svn: 313741