PTH: Cache stat information for files in the PTH file. Hook up FileManager

 to use this stat information in the PTH file using a 'StatSysCallCache' object.

Performance impact (Cocoa.h, PTH):
- number of stat calls reduces from 1230 to 425
- fsyntax-only: time improves by 4.2% 

We can reduce the number of stat calls to almost zero by caching negative stat
calls and directory stat calls in the PTH file as well.

llvm-svn: 64353