[codeview] Add TypeVisitorCallbackPipeline.

We were kind of hacking this together before by embedding the
ability to forward requests into the TypeDeserializer.  When
we want to start adding more different kinds of visitor callback
interfaces though, this doesn't scale well and is very inflexible.

So introduce the notion of a pipeline, which itself implements
the TypeVisitorCallbacks interface, but which contains an internal
list of other callbacks to invoke in sequence.

Also update the existing uses of CVTypeVisitor to use this new
pipeline class for deserializing records before visiting them
with another visitor.

llvm-svn: 280293