[ORC] Allow EPCEHFrameRegistrar clients to specify registration function dylib.

Previously, EPCEHFrameRegistrar always used the
ExecutorProcessControl::loadDylib(nullptr) method to obtain a handle for the
process, but this doesn't work if the registration functions aren't visible in
a standard search of the process (e.g. if the JIT is in a plugin that is loaded
with RTLD_LOCAL).

This patch retains the old behavior by default, but allows clients to supply
their own handle for the library containing the registration functions if they
need to (e.g. to work around limitations like RDLD_LOCAL above, which aren't
expressible within the existing loadDylib / DynamicLibrary APIs).