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
diff --git a/llvm/lib/IR/LLVMContextImpl.cpp b/llvm/lib/IR/LLVMContextImpl.cpp
index 1e20807..0ce2871 100644
--- a/llvm/lib/IR/LLVMContextImpl.cpp
+++ b/llvm/lib/IR/LLVMContextImpl.cpp
@@ -78,7 +78,7 @@
   // unnecessary RAUW when nodes are still unresolved.
   for (auto *I : DistinctMDNodes)
     I->dropAllReferences();
-#define HANDLE_MDNODE_LEAF(CLASS)                                              \
+#define HANDLE_MDNODE_LEAF_UNIQUABLE(CLASS)                                    \
   for (auto *I : CLASS##s)                                                     \
     I->dropAllReferences();
 #include "llvm/IR/Metadata.def"
@@ -92,8 +92,8 @@
   // Destroy MDNodes.
   for (MDNode *I : DistinctMDNodes)
     I->deleteAsSubclass();
-#define HANDLE_MDNODE_LEAF(CLASS)                                              \
-  for (CLASS *I : CLASS##s)                                                    \
+#define HANDLE_MDNODE_LEAF_UNIQUABLE(CLASS)                                    \
+  for (CLASS * I : CLASS##s)                                                   \
     delete I;
 #include "llvm/IR/Metadata.def"