IR: Remove MDNodeFwdDecl

Remove `MDNodeFwdDecl` (as promised in r226481).  Aside from API
changes, there's no real functionality change here.
`MDNode::getTemporary()` now forwards to `MDTuple::getTemporary()`,
which returns a tuple with `isTemporary()` equal to true.

The main point is that we can now add temporaries of other `MDNode`
subclasses, needed for PR22235 (I introduced `MDNodeFwdDecl` in the
first place because I didn't recognize this need, and thought they were
only needed to handle forward references).

A few things left out of (or highlighted by) this commit:

  - I've had to remove the (few) uses of `std::unique_ptr<>` to deal
    with temporaries, since the destructor is no longer public.
    `getTemporary()` should probably return the equivalent of
    `std::unique_ptr<T, MDNode::deleteTemporary>`.
  - `MDLocation::getTemporary()` doesn't exist yet (worse, it actually
    does exist, but does the wrong thing: `MDNode::getTemporary()` is
    inherited and returns an `MDTuple`).
  - `MDNode` now only has one subclass, `UniquableMDNode`, and the
    distinction between them is actually somewhat confusing.

I'll fix those up next.

llvm-svn: 226501

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -541,7 +541,7 @@ void BitcodeReaderMDValueList::AssignValue(Metadata *MD, unsigned Idx) {
   }
 
   // If there was a forward reference to this value, replace it.
-  MDNodeFwdDecl *PrevMD = cast<MDNodeFwdDecl>(OldMD.get());
+  MDTuple *PrevMD = cast<MDTuple>(OldMD.get());
   PrevMD->replaceAllUsesWith(MD);
   MDNode::deleteTemporary(PrevMD);
   --NumFwdRefs;
@@ -573,9 +573,12 @@ void BitcodeReaderMDValueList::tryToResolveCycles() {
 
   // Resolve any cycles.
   for (auto &MD : MDValuePtrs) {
-    assert(!(MD && isa<MDNodeFwdDecl>(MD)) && "Unexpected forward reference");
-    if (auto *N = dyn_cast_or_null<UniquableMDNode>(MD))
-      N->resolveCycles();
+    auto *N = dyn_cast_or_null<UniquableMDNode>(MD);
+    if (!N)
+      continue;
+
+    assert(!N->isTemporary() && "Unexpected forward reference");
+    N->resolveCycles();
   }
 }