Fix add_llvm_external_project.
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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@251001 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm