[CMake] Make the bindir and libdir arguments to set_output_directory optional

When building a plugin against an installed LLVM toolchain using
add_llvm_loadable_module (in the documented manner) doesn't work as nothing sets
the *_OUTPUT_INTDIR variables causing an error when set_output_directory is
called. Making those arguments optional (causing the default output directory
to be used) fixes this.

Differential Revision: http://reviews.llvm.org/D13215

llvm-svn: 248911