Cache size per class size in SizeClassAllocatorXLocalCache.

Summary:
Allocator::ClassIdToSize() is not free and calling it in every
Allocate/Deallocate has noticeable impact on perf.
Reapplying D31991 with the appropriate fixes.

Reviewers: cryptoad

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D32024

llvm-svn: 300216