alltogether: Handle missing target information more gracefully

[dtzWill]

Currently, an error like the following is printed:

Combining referenced modules in allexe
  Input: /home/will/allvm-nixpkgs/allvm/tmp/binary-decompilation-6efd921/test/decompilation/test_0/Output/test_0.clang.trans.allexe
  Output: test_0.clang.trans.unified.allexe
Extracting and merging modules...
  main.bc:
    Loading...Adding...Success.
  Output//ELF_64_linux.bc:
    Loading...Assertion failed: T && T->hasMCAsmParser() (/tmp/nix-build-llvm-all-master-df71b60.drv-0/llvm/lib/Object/ModuleSymbolTable.cpp: CollectAsmSymbols: 68)
Segmentation fault

Which is not very helpful.

Ideas for solution:

• Detect missing data layout/triple information, print nice error. Perhaps check for this in bc2allvm.
• Detect missing data layout/triple information, print warning, set triple to native target.

Example will be submitted shortly.