[asan] introduce two functions that will allow implementations of C++ garbage colection to work with asan's fake stack

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200908 91177308-0d34-0410-b5e6-96231b3b80d8
[asan] fix testing on Mac

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200910 91177308-0d34-0410-b5e6-96231b3b80d8