swift234

Reflection: Move field and associated type metadata parsing from ReflectionContext to TypeRefBuilder, NFC

ReflectionContext is now solely concerned with converting runtime
metadata in a remote address space into a TypeRef.

TypeRefBuilder now knows how to parse reflection metadata, and
in particular look up associated type witnesses. This decouples
the TypeRef substitution code from the ReflectionContext. Now
substitution only needs a TypeRefBuilder, which means the code
is no longer templated, and can be moved from TypeRef.h to
TypeRef.cpp.

This also allows the upcoming TypeRef lowering code to live in
a source file instead of headers.