Port my recent changes from LLVM copy of the demangler:

r340663 - Allow Allocator::make to make a node of a different type than that
          requested.
r340664 - Add documentation comment to ForwardTemplateReference.
r340665 - Fix ExpandedSpecialSubstitution demangling for Sa and Sb.
r340670 - Allow demangler's node allocator to fail, and bail out of the entire
          demangling process when it does.

llvm-svn: 340671