Ensure that all module build failures get diagnosed

Otherwise we can end up silently skipping an import.  If we happen to be
building another module at the time, we may build a mysteriously broken
module and not know why it seems to be missing symbols.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218552 91177308-0d34-0410-b5e6-96231b3b80d8