Implement the LOADABLE_MODULE option when building a shared library. This

passes the -module option on the libtool command line to ensure that the
shared library being built can be dlopened and dlsym can work on that
module. LOADABLE_MODULE should be sent only in conjunction with the
SHARED_LIBRARY directive. It should generally be used for any module that
is intended to be the target of an LLVM -load option. Note that loadable
modules will not have the lib prefix but otherwise look like shared
libraries. This is per the libtool recommendations and prevents these
special shared libraries from being linked in via -l option to the linker.

llvm-svn: 19454