[CMake] Support lld with LTO bootstrap

lld has LTO support, if requested we should add a dependency on lld
rather than LLVMgold when doing LTO bootstrap build.

Differential Revision: https://reviews.llvm.org/D26649

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287179 91177308-0d34-0410-b5e6-96231b3b80d8