This is a follow up patch that allows for external DescriptorFinders to
provide MultiPayloadEnumDescriptors (this is done to support embedded
Swift debugging, which encodes the equivalent of Swift metadata as
DWARF).
Currently, TypeRefBuilder knows how to parse type descriptors from
reflection metadata. We aim to store the same information that lives
in type descriptors externally, as an effort to support embedded Swift
debugging. In order to take advantage of all of the existing type
information parsing code that exists today in remote mirrors, add an
interface for external lookup of type descriptors, and replace all the
usages of type descriptors with their generic counterpart (this patch
only adds support for builtin descriptors, but follow up patches will
add support for other descriptor types).