DI: Disallow uniquable DICompileUnits
Since r241097, DIBuilder has only created distinct DICompileUnits.
The backend is liable to start relying on that (if it hasn't already),
so make uniquable DICompileUnits illegal and automatically upgrade old
bitcode. This is a nice cleanup, since we can remove an unnecessary
DenseSet (and the associated uniquing info) from LLVMContextImpl.
Almost all the testcases were updated with this script:
git grep -e '= !DICompileUnit' -l -- test | grep -v test/Bitcode | xargs sed -i '' -e 's,= !DICompileUnit,= distinct !DICompileUnit,'
I imagine something similar should work for out-of-tree testcases.