Missing -Wreturn-stack-address cases for lambda.

[hokein]

#include <algorithm>
#include <functional>

auto foo(int x) {
    return [&]() { return x + 1; }; // good, give a warning.
}
std::function<int()> foo2(int x) {
    return [&]() { return x + 1; }; // no warning
}

auto foo3(int x) {
    auto a = [&]() { return x + 1;};
    return a; // no warning.
}

https://godbolt.org/z/nfacaEfGM, it would be good to detect case 2 and case 3.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

``` #include <algorithm> #include <functional>

auto foo(int x) {
return & { return x + 1; }; // good, give a warning.
}
std::function<int()> foo2(int x) {
return & { return x + 1; }; // no warning
}

auto foo3(int x) {
auto a = & { return x + 1;};
return a; // no warning.
}

https://godbolt.org/z/nfacaEfGM, it would be good to detect case 2 and case 3.
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