swift234

Debugging on SIL level.

This change follows up on an idea from Michael (thanks!).
It enables debugging and profiling on SIL level, which is useful for compiler debugging.

There is a new frontend option -gsil which lets the compiler write a SIL file and generated debug info for it.
For details see docs/DebuggingTheCompiler.rst and the comments in SILDebugInfoGenerator.cpp.