[sanitizer][NFC] Get type of AllocatorCache from CombinedAllocator

Reviewers: eugenis, cryptoad, kcc

Reviewed By: kcc

Subscribers: kcc, kubamracek, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61155

llvm-svn: 359715