[ORC] Add support for dumping objects to the C API.

Provides ObjectTransformLayer APIs, a getter to access the
ObjectTransformLayer member of LLJIT, and the DumpObjects utility
to make construction of a dump-to-disk transform easy.

An example showing how the new APIs can be used has been added in
llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects.