[dwarfdump] Skip 'stripped' sections

When dsymutil generates the companion file, its strips all unnecessary
sections by omitting their body and setting the offset in their
corresponding load command to zero.

One such section is the .eh_frame section, as it contains runtime
information rather than debug information and is part of the __TEXT
segment. When reading this section, we would just read the number of
bytes specified in the load command, starting from offset 0 (i.e. the
beginning of the file).

Rather than trying to parse this obviously invalid section, dwarfdump
now skips this.

Differential revision: https://reviews.llvm.org/D38135

llvm-svn: 314208