[ORC] Add custom object interface support to StaticLibaryDefinitionGenerator.

This adds a GetObjectFileInterface callback member to
StaticLibraryDefinitionGenerator, and adds an optional argument for initializing
that member to StaticLibraryDefinitionGenerator's named constructors. If not
supplied, it will default to getObjectFileInterface from ObjectFileInterface.h.

To enable testing a `-hidden-l<x>` option is added to the llvm-jitlink tool.
This allows archives to be loaded with all contained symbol visibilities demoted
to hidden.

The ObjectLinkingLayer::setOverrideObjectFlagsWithResponsibilityFlags method is
(belatedly) hooked up, and enabled in llvm-jitlink when `-hidden-l<x>` is used
so that the demotion is also applied at symbol resolution time (avoiding any
"mismatched symbol flags" crashes).