[CMake] Add the gold plugin before clang
[CMake] Add the gold plugin before clang

This is needed to connect dependencies between the LLVMgold plugin and the clang stage-2 builds due to limitations in ExternalProject_Add.

Patch by Mike Edwards

Differential Revision: http://reviews.llvm.org/D17655

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@262067 91177308-0d34-0410-b5e6-96231b3b80d8

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