[cs] pipe infrastructure support

[Benjamin Kosnik]

> I guess you don't want a c++ compiler that is 2.2-2.5x faster do you 
> then?  Some people actually care about compile speed.  Hopefully the 
> people that review this won't actually stand in the way of progress.  
> If you have a better way, put it in now, otherwise...

I know! How about making the last piece of Apple-contributed work that
"makes the c++ compiler 2-2.5x faster" actually work again?

pch/12112?

If these compile-server bits are anything like the PCH support, Neil is
right to be concerned about long-term maintainability: there is
effectively no maintenance of PCH. Why is that? Has Apple decided that
the compile server is a better way to go? Or, does PCH work on darwin,
and is just broken everywhere else? Do tell.

curious,
benjamin