[CMake] Add lto subdirectory explicitly.

This is required because ExternalProject_Add requires all targets specified in the DEPENDS argument must exist before calling ExternalProject_Add.

I have a follow-up patch to clang that enables using the just-built libLTO in bootstrap builds, so we need to be able to add the LTO target as a dependency in clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247316 91177308-0d34-0410-b5e6-96231b3b80d8