[llvm-dwarfdump] Add -o to help text and remove --out-file from doc

-o is in the documentation, but not in the llvm-dwarfdump help text.
This patch adds it by inverting the -o and --out-file aliasing. It also
removes --out-file from the documentation, since we don't really want
people to be using this switch in practice.

Reviewed by: aprantl, JDevlieghere, dblaikie

Differential Revision: https://reviews.llvm.org/D63013

llvm-svn: 363044