Remove stat cache chaining as it's no longer needed after PTH support has been

removed

Stat cache chaining was implemented for a StatListener in the PTH writer so that
it could write out the stat information to PTH. r348266 removed support for PTH,
and it doesn't seem like there are other uses of stat cache chaining. We can
remove the chaining support.

Differential Revision: https://reviews.llvm.org/D55455

llvm-svn: 349942