cmake: Use "set" instead of "option" for LLVM_ENABLE_LTO
cmake: Use "set" instead of "option" for LLVM_ENABLE_LTO

Apparently option is for bools and cmake-gui will display this
strangely with option.

Pointed out by edward-san - thanks!

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@260154 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm