[LLJIT] Add convenience methods for loading dylibs and linking static libs.

[LLJIT] Add convenience methods for loading dylibs and linking static libs.

LLJIT::loadPlatformDynamicLibrary loads a dynamic library at a given path
(interpreted in the executor process -- the process containing the JIT'd code),
and returns a JITDylib (whose name is the given path) that reflects the symbols
in that library. LLJIT clients wishing to make the given symbols visible to
their JIT'd code can add this JITDylib to the link order of their JITDylib(s)
using JITDylib::addToLinkOrder.

The LLJIT::linkStaticLibraryInto overloads load a static library (or universal
binary) at a given path (interpreted in the controller process -- the process
containing the LLJIT instance) and adds its symbols to the given JITDylib.

The lli tool is updated to use LLJIT::linkStaticLibraryInto to implement the
extra-archive option.

LLJIT::loadPlatformDynamicLibrary is not tested in this patch as we don't have
a good way to produce dylibs in LLVM's regression test suite.