llvm-dwarfdump: automatically dump both regular and .dwo variant of sections

Since users typically don't really care about the .dwo / non.dwo
distinction, this patch makes it so dwarfdump --debug-<info,...> dumps
.debug_info and (if available) also .debug_info.dwo. This simplifies
the command line interface (I've removed all dwo-specific dump
options) and makes the tool friendlier to use.

Differential Revision: https://reviews.llvm.org/D37771

llvm-svn: 313207