[ORC] Refactor definition-generation, add a generator for static libraries.

This patch replaces the JITDylib::DefinitionGenerator typedef with a class of
the same name, and adds support for attaching a sequence of DefinitionGeneration
objects to a JITDylib.

This patch also adds a new definition generator,
StaticLibraryDefinitionGenerator, that can be used to add symbols fom a static
library to a JITDylib. An object from the static library will be added (via
a supplied ObjectLayer reference) whenever a symbol from that object is
referenced.

To enable testing, lli is updated to add support for the --extra-archive option
when running in -jit-kind=orc-lazy mode.

llvm-svn: 368707