Casting a pointer to a different address space is a nice trick to
prevent an access from being instrumented by sanitizers such as ASan.
However, this trick is currently broken, as the test demonstrates.
A minimal C PoC of the issue is:
```
void access(void *p) {
auto tmp = (unsigned long __attribute__((address_space(1))) volatile *)p;
*tmp = 0;
}
```
Under fsanitize=address, the access does not get instrumented, but under
fsanitize=thread, it does.
(Note: Before this patch, this test will actually hit a CallInst
assertion if you have them turned on, because the TSan runtime functions
are defined to take pointer arguments, and non-default address space
pointers are not compatible).
rdar://166743781
99e8aa5ab4