cmake: Add a flag to enable LTO

This adds -DLLVM_ENABLE_LTO, rather than forcing people to manually
add -flto to the various _FLAGS variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259766 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed