Add a comment explaining how the input for...

Add a comment explaining how the input for GetModuleSpecifications_EarlySectionHeaders was generated

Davide pointed out this would be useful if the file ever needs to be
regenerated (and I certainly agree).

I also replace the test binary with a slightly smaller one -- I intended
to do this in the original commit, but I forgot to add it to the patch
as I was juggling several things at the same time.

llvm-svn: 324256