[CMake] Separate LLVM_ENABLE_LTO and LLVM_ENABLE_LLD

These two are independent: it's possible to use LLD without LTO,
and it's possible to do LTO build without LLD.

Differential Revision: https://reviews.llvm.org/D28821

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292343 91177308-0d34-0410-b5e6-96231b3b80d8