[asan][StackSafety] Custom poisoning is no-op with -asan-use-stack-safety=1

[vitalybuka]

Stack safety does not take into account that we can partially poison alloca.
This may remove mem access instrumentation and not detect bugs detectable with -asan-use-stack-safety=0.

https://godbolt.org/z/ndzbEadM8

#include <sanitizer/asan_interface.h>
#include <iostream>

__attribute__((noinline, weak))
void store(long long& b) {
    __asan_poison_memory_region(&b, sizeof(b));
}

__attribute__((noinline, weak))
long long test() {
    long long b;
    store(b);
    return b;
}

int main() {
    test();
}