swift234

Rename DeadCodeElimination.cpp to DiagnoseUnreachable.cpp.

Also rename dead_code_elimination.sil to diagnose_unreachable.sil.

This more accurately reflects what this file implements, and makes way
for adding a new DeadCodeElimination.cpp that does classic dead (as
opposed to unreachable) code elimination.

I will fix up the contents of the files as a separate step.

Swift SVN r18613