ThinLTO: don't perform incremental LTO on module without a hash

Clang always emit a hash for ThinLTO, but as other frontend are
starting to use ThinLTO, this could be a serious bug.

Differential Revision: https://reviews.llvm.org/D25379

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283655 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed