[BUG] -Wunreachable-code not working

HI all,

I'm reporting this bug here since the LLVM Bugzilla requires
registering an account (which can't be done by myself manually).

Basically, this code triggers the warning:

int main()
{
    return 0;

    int a = 42;
}

while this doesn't:

#define A 42

int main()
{
    return 0;

    int a = A;
}

This happens on clang++-6.0. I'm not using any options other than
-Wunreachable-code.