[llvm-dwp] Implement -e option

The binutils utility dwp has an option "-e" 
https://gcc.gnu.org/wiki/DebugFissionDWP
to specify an executable/library to get the list 
of *.dwo files from it. This option is particularly useful when 
someone runs the tool manually outside of a build system.
This diff adds an implementation of "-e" to llvm-dwp.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D37371

llvm-svn: 312409