[ORC] Start migrating ORC layers to use the new ORC Core.h APIs.

In particular this patch switches RTDyldObjectLinkingLayer to use
orc::SymbolResolver and threads the requried changse (ExecutionSession
references and VModuleKeys) through the existing layer APIs.

The purpose of the new resolver interface is to improve query performance and
better support parallelism, both in JIT'd code and within the compiler itself.

The most visibile change is switch of the <Layer>::addModule signatures from:

Expected<Handle> addModule(std::shared_ptr<ModuleType> Mod,
                           std::shared_ptr<JITSymbolResolver> Resolver)

to:

Expected<Handle> addModule(VModuleKey K, std::shared_ptr<ModuleType> Mod);

Typical usage of addModule will now look like:

auto K = ES.allocateVModuleKey();
Resolvers[K] = createSymbolResolver(...);
Layer.addModule(K, std::move(Mod));

See the BuildingAJIT tutorial code for example usage.

llvm-svn: 324405