swift234

[cmake] Add an option to compile the compiler with -flto.

This doesn't hit the runtime. I am going to try that in a little bit.

I saw some nice reductions in compile time in the SILOptimizer in the stdlib.
Specifically, we went from spending ~33 seconds in the siloptimizer -> ~23
seconds. There were other improvements as well.

Keep in mind, I have not tested this with debug info, so it may hog all of your
ram due to crazy amounts of debug info. It would be good to try and do what LLVM
does which is to compile with line tables but not full types.

rdar://24717107