Factor lsan allocator cache accesses into a function

Summary:
This patch is the first step towards allows us to move away from using
__thread for the allocator cache on darwin,
which is requiring for building lsan for darwin on ios version 7
and on iossim i386.

This will be followed by patches to move the function into OS-specific files.

Reviewers: kubamracek, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29994

llvm-svn: 298537