Sema was dutifully tracking conformances that were "used" as part of
type checking, so it could make sure that those conformances got
completed for SILGen to use. However, this information never actually
made it to SILGen, which included its own (more conservative, not
broad enough) heuristics for finding "used" conformances. Teach Sema
to record conformances within the appropriate source file, and have
SILGen reference the conformances when it emits SIL for the source
file.
The interesting thing here is that we need runtime support in
order to generate references to metatypes for classes, mostly
because normal ObjC classes don't have all the information we want
in a metatype (which for now just means the VWT pointer).
We'll need to be able to reverse this mapping when finding a
class pointer to hand off to, say, an Objective-C class method,
of course.
Swift SVN r3424
