[asan] attempting to fix the Mac build

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172725 91177308-0d34-0410-b5e6-96231b3b80d8
ASan: Allow __asan_get_allocated_size(malloc(0)) to be 0 or 1

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172726 91177308-0d34-0410-b5e6-96231b3b80d8
[ASan] minor changes to swapcontext handling: don't clear shadow memory if context stack is too large

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172727 91177308-0d34-0410-b5e6-96231b3b80d8