Address sanitizer recently got a new macro __SANITIZER_DISABLE_CONTAINER_OVERFLOW__
which is intended to disable container overflow checks in libraries (either the
standard library or user libraries that might provide such checks). This patch makes
libc++ honor that macro and, in addition, cleans up how these checks are enabled for
string (which is special) by introducing a macro just for string.
rdar://166234942
20014885c3