CodeGen: Replace test/CodeGen/thinlto_backend.c with a functional test.

This new test tests that functions are capable of being imported, rather than
that the import pass is run. This new test is compatible with the approach
being developed in D20268 which runs the importer on its own rather than in
a pass.

Differential Revision: http://reviews.llvm.org/D21542

llvm-svn: 273347