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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64353 91177308-0d34-0410-b5e6-96231b3b80d8