[ELF] - LTO: add test for createEmptyIndex.

We have a following comment for createEmptyIndex caller code:

// If LazyObjFile has not been added to link, emit empty index files.
// This is needed because this is what GNU gold plugin does and we have a
// distributed build system that depends on that behavior.
  
Though createEmptyIndex() itself
(https://github.com/llvm-mirror/lld/blob/master/ELF/LTO.cpp#L202)
is never called in our test cases. 

Patch adds a test.

llvm-svn: 336270