swift234

Add a new frontend option for debugging called '-disable-incremental-llvm-codegen'.

Currently IRGen stores hashes of the bitcode generated by swift in object files.
This is then used to reduce compile time by not re-codegening if a subsequent
compilation yields a bit code with the same hash.

This is good for users and general compilation, but can result in confusion when
attempting to measure the "real" compile time of the compiler.

By default it is off.