[LSan] Factor lsan allocator cache accesses into a function

[LSan] 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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@298537 91177308-0d34-0410-b5e6-96231b3b80d8

Backport:
  http://llvm.org/git/compiler-rt 65ede46c

Change-Id: Icd528cc4e96c63489c7d25cbb3af100a32d4a26f
Signed-off-by: Vyacheslav Cherkashin <v.cherkashin@samsung.com>