Define a new macro in libc++ named '_LIBCPP_HAS_NO_ASAN'. When this is defined,

libc++ will not call address_sanitizer to detect addressing errors in the
standard library containers. This is a negative macro to enable users to
disable the libc++ checks even if they are compiling with address sanitizer
enabled by defining this macro.

At the present time, there is no code in libc++ that looks at this macro.
That will come soon. This is just infrastructure.

llvm-svn: 206184