Add lightweight shim "clang::StatSysCallCache" that caches 'stat' system calls

for use by FileManager. FileManager now takes a StatSysCallCache* in its
constructor (which defaults to NULL). This will be used for evaluating whether
or not caching 'stat' system calls in PTH is a performance win. This shim adds
no observable performance impact in the case where the 'StatSysCallCache*' is
null.

llvm-svn: 64345