[llvm] [CMake] Set LLVM_USE_LINKER for external projects when using lld (PR #170401)

[Zachary Fogg via llvm-commits]

zfogg wrote:

@petrhosek @smeenai Could you review this? This fixes a build failure when using LTO with runtimes - clang wasn't using lld because CMAKE_LINKER isn't respected on Linux.

https://github.com/llvm/llvm-project/pull/170401