This API will take a pointer to the start of an existential
container and its typeref and provide a typeref for its
instance type and the address to the start of the instance's
data.
This has a dummy implementation that returns false for now.
swift_reflection_createReflectionContext was defaulting to C++ mangling
because the declaration under extern C in the header didn't match
its signature.
In order to perform layout, the remote mirrors library needs to know
about the size, alignment and extra inhabitants of builtin types.
Ideally we would emit a reflection info section in libswiftRuntime.o,
but in the meantime just duplicate builtin type metadata for all
builtin types referenced from the current module instead.
In practice only the stdlib and a handful of overlays like the SIMD
overlay use builtin types, and only a few at a time.
Tested manually by running swift-reflection-tool on the standard
library -- I'll add automated tests by using -parse-stdlib to
reference Builtin types in a subsequent patch that adds more layout
logic.
NFC if -enable-reflection-metadata is off.
