Revert "Revert "ThinLTO: Verify bitcode before lauching the ThinLTOCodeGenerator.""

This reapplies commit r303438 modified to not verify cross-imported
bitcode in FunctionImporter.

rdar://problem/31233625

Differential Revision: https://reviews.llvm.org/D33370

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303470 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed