Fix `skipIfSanitized` decorator on macOS

For security reasons, DYLD_INSERT_LIBRARIES is not propagated to a child
process. This breaks the skipIfSanitized decorator, which checks for the
environment variable being set. Instead, always set the ASAN_OPTIONS and
make the decorator check for that.

Differential revision: https://reviews.llvm.org/D65594

llvm-svn: 367595