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.
In particular, this will allow the field and associated type metadata
parsing to be moved out of ReflectionContext and into TypeRefBuilder,
which is required for TypeRef lowering.
