swift234

[cmake] Add the option SWIFT_BUILD_RUNTIME_WITH_HOST_LIBLTO.

Currently when we build with bitcode, we use the liblto that we just built,
despite the fact that we just compiled the runtime with the host clang. Really,
we should be consistent.

In the short term to unblock LTO (which wants to use the host liblto), provide
an option to use the host liblto but leave it off so that the normal behavior is
preserved.