DI: Disallow uniquable DICompileUnits

Since r241097, `DIBuilder` has only created distinct `DICompileUnit`s.
The backend is liable to start relying on that (if it hasn't already),
so make uniquable `DICompileUnit`s 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.

llvm-svn: 243885

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -943,7 +943,8 @@ static void WriteDICompileUnit(const DICompileUnit *N,
                                BitstreamWriter &Stream,
                                SmallVectorImpl<uint64_t> &Record,
                                unsigned Abbrev) {
-  Record.push_back(N->isDistinct());
+  assert(N->isDistinct() && "Expected distinct compile units");
+  Record.push_back(/* IsDistinct */ true);
   Record.push_back(N->getSourceLanguage());
   Record.push_back(VE.getMetadataOrNullID(N->getFile()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawProducer()));