[DWARFv5] Verify all-or-nothing constraint on DIFile source

Update IR verifier to check the constraint that DIFile source is present on all
files or no files.

Differential Revision: https://reviews.llvm.org/D54953

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348022 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed