[llvm-dwarfump] Don't rely on global state, part 3.

[llvm-dwarfump] Don't rely on global state, part 3.

Some tools used to rely on a global static variable to keep track of the
return value for main(). I changed llvm-cxxdump to use exit(1)
and Rafael shortly after did the same with llvm-readobj. This is
(yet) another step towards the goal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243240 91177308-0d34-0410-b5e6-96231b3b80d8