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

[Petr Hosek via llvm-commits]

petrhosek wrote:

`llvm_ExternalProject_Add` is used for different subprojects (for example runtimes) and not all of those recognize the `LLVM_USE_LINKER` option. I think that `LLVM_USE_LINKER=lld` should be set by the caller of `llvm_ExternalProject_Add` as needed.

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