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

[Zachary Fogg via llvm-commits]

zfogg wrote:

i have to manually add `-DLLVM_USE_LINKER=lld` right now and I shouldn't have to. if I don't have `-DLLVM_USE_LINKER=lld` I get the build error described above.

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