extend `returns_nonnull` function attribute diagnostics

[firewave]

__attribute__((returns_nonnull)) void* func();

static void f1()
{
    if (func()) {} // warning
}

static void f2()
{
    void* p = func();
    if (p) {} // no warning
}

<source>:5:9: warning: nonnull function call 'func()' will evaluate to 'true' on first encounter [-Wpointer-bool-conversion]
    5 |     if (func()) {} // warning
      |     ~~  ^~~~~~
<source>:1:16: note: declared 'returns_nonnull' here
    1 | __attribute__((returns_nonnull)) void* func();
      |  

https://godbolt.org/z/x3r5oP6db

I guess this is out of scope of the existing warning and would require a clang-tidy or static analyzer check instead.

[firewave]

Also no warning for variable declaration in the if-condition:

[[gnu::returns_nonnull]] char* cb();

void f()
{
    if (char* c = cb()) {}
}

https://godbolt.org/z/119Gaff37