The code we use to interface with the platform dynamic linker is turning into a rat's nest of conditionals that's hard to maintain and extend. Since ELF, Mach-O, and PE platforms have pretty fundamentally different dynamic linker interfaces and capabilities, it makes sense to factor that code into a separate file per-platform, instead of trying to conditionalize the logic in-line. This patch factors out a much simpler portable interface for lazily kicking off the protocol conformance and type metadata lookup caches, and factors the guts out into separate MachO, ELF, and Win32 backends. This should also be a much cleaner interface to interface static binary behavior into, assisting swiftlang#5349.