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.

Reviewers: eugenis, kcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D31991

llvm-svn: 300107