[demangler] Use a back-patching scheme to resolve forward references.

Strictly in a conversion operator's type, a <template-param> refers to a
<template-arg> that is further ahead in the mangled name. Instead of
doing a second parse to resolve these, introduce a
ForwardTemplateReference Node and back-patch the referenced
<template-arg> when we're in the right context.

This is also a correctness fix, previously we would only do a second
parse if the <template-param> was out of bounds in the current set of
<template-args>. This lead to misdemangles (gasp!) when the conversion
operator was a member of a templated struct, for instance.

llvm-svn: 328464