cmake: Accept "thin" or "full" as arguments to -DLLVM_ENABLE_LTO

Mehdi suggested in a review of r259766 that it's also useful to easily
set the type of LTO. Augment the cmake variable to support that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260143 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed