llvm-dwarfdump: Replace -debug-dump=sect option with individual options.

Description

llvm-dwarfdump: Replace -debug-dump=sect option with individual options.

As discussed on llvm-dev in
http://lists.llvm.org/pipermail/llvm-dev/2017-September/117301.html
this changes the command line interface of llvm-dwarfdump to match the
one used by the dwarfdump utility shipping on macOS. In addition to
being shorter to type this format also has the advantage of allowing
more than one section to be specified at the same time.

In a nutshell, with this change

$ llvm-dwarfdump --debug-dump=info
$ llvm-dwarfdump --debug-dump=apple-objc

becomes

$ dwarfdump --debug-info --apple-objc

Differential Revision: https://reviews.llvm.org/D37714