Fix add_llvm_external_project.

r250835 unintentionally discarded the optional parameter to the
add_llvm_external_project() macro that may point to a path when the said
path is different from ${name}. This should fix it by passing ${ARGN} on
to add_llvm_subdirectory(). The problem manifests itself with e.g.
add_llvm_external_project(clang-tools-extra extra) from
clang/tools/CMakeLists.txt

Patch by Luchesar V. Iliev.

llvm-svn: 251001