Reapply "[dwarfdump] Add support for dumping accelerator tables."

This reverts commit r221842 which was a revert of r221836 and of the
test parts of r221837.

This new version fixes an UB bug pointed out by David (along with
addressing some other review comments), makes some dumping more
resilient to broken input data and forces the accelerator tables
to be dumped in the tests where we use them (this decision is
platform specific otherwise).

llvm-svn: 222003