This adds the argument --dump-ir to clang-import-test, which allows

viewing of the final IR. This is useful for confirming that 
structure layout was correct.

I've added two tests:

- A test that checks that structs in top-level code are completed 
  correctly during struct layout (they are)
- A test that checks that structs defined in function bodies are 
  cpmpleted correctly during struct layout (currently they are not, 
  so this is XFAIL).

The second test fails because LookupSameContext()
(ExternalASTMerger.cpp) can't find the struct. This is an issue I 
intend to resolve separately.

Differential Revision: https://reviews.llvm.org/D36429

llvm-svn: 310318