Fix r357749 for big-endian architectures

We need to read the strings from the minidump files as little-endian,
regardless of the host byte order.

I definitely remember thinking about this case while writing the patch
(and in fact, I have implemented that for the "write" case), but somehow
I have ended up not implementing the byte swapping when reading the
data. This adds the necessary byte-swapping and should hopefully fix
test failures on big-endian bots.

llvm-svn: 357754