IR: DwarfNode => DebugNode, NFC

These things are potentially used for non-DWARF data (see the discussion
in PR22235), so take the `Dwarf` out of the name.  Since the new name
gives fewer clues, update the doxygen to properly describe what they
are.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226874 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/AsmWriter.cpp

@@ -1286,7 +1286,7 @@ raw_ostream &operator<<(raw_ostream &OS, FieldSeparator &FS) {
 }
 } // end namespace
 
-static void writeGenericDwarfNode(raw_ostream &, const GenericDwarfNode *,
+static void writeGenericDebugNode(raw_ostream &, const GenericDebugNode *,
                                   TypePrinting *, SlotTracker *,
                                   const Module *) {
   llvm_unreachable("Unimplemented write");