ASAN and/or UBSAN missing cases depending on optimization level

[janwilmans]

https://cppcoach.godbolt.org/z/34TPh8vPh

#include <vector>

char example(const std::vector<char> v)
{
    return v[2];
}

int main()
{
    std::vector<char> values;
    values.resize(10);
    values.resize(1);
    example(values);
}

observations:

• gcc14 catches this case with -fsanitize=address,undefined at all levels (just as an aside)
• gcc14 misses this case with just -fsanitize=address at all levels above -O0 (just as an aside)
• clang18 catches this case with -fsanitize=address at -O0
• clang18 misses this case with -fsanitize=address at -O1 or higher, and adding -fsanitize=address,undefined makes no difference.