IR: Handle self-referencing DICompositeTypes in DIBuilder

Add API to DIBuilder to handle self-referencing `DICompositeType`s.

Self-references aren't expected in the debug info graph, and we take
advantage of that by only calling `resolveCycles()` on nodes that were
once forward declarations (otherwise, DIBuilder needs an expensive
tracking reference to every unresolved node it creates, which in cyclic
graphs is *all of them*).

However, clang seems to create self-referencing `DICompositeType`s.  Add
API to manage this safely.  The paired commit to clang will include the
regression test.

I'll make the `DICompositeType` API `private` in a follow-up to prevent
misuse (I've separated that to prevent build failures from missing the
clang commit).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224482 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DIBuilder.h

@@ -698,6 +698,20 @@ namespace llvm {
     Instruction *insertDbgValueIntrinsic(llvm::Value *Val, uint64_t Offset,
                                          DIVariable VarInfo, DIExpression Expr,
                                          Instruction *InsertBefore);
+
+    /// \brief Replace the vtable holder in the given composite type.
+    ///
+    /// If this creates a self reference, it may orphan some unresolved cycles
+    /// in the operands of \c T, so \a DIBuilder needs to track that.
+    void replaceVTableHolder(DICompositeType &T, DICompositeType VTableHolder);
+
+    /// \brief Replace arrays on a composite type.
+    ///
+    /// If \c T is resolved, but the arrays aren't -- which can happen if \c T
+    /// has a self-reference -- \a DIBuilder needs to track the array to
+    /// resolve cycles.
+    void replaceArrays(DICompositeType &T, DIArray Elements,
+                       DIArray TParems = DIArray());
   };
 } // end namespace llvm