[[clang::lifetimebound]] doesn't work with default arguments

[higher-performance]

const int& foo(const int& x [[clang::lifetimebound]] = 0) {
    return x;
}
int main() {
    const int& x = foo();  // No error
    return x;
}

[shafik]

I thought maybe this was working as intended since the lifetime of the default param would be in the function that called foo but an extended example also shows this behavior: https://godbolt.org/z/G1b9b1Ez6

CC @zygoloid who implemented the feature via f4e248c

[llvmbot]

@llvm/issue-subscribers-clang-frontend

``` const int& foo(const int& x [[clang::lifetimebound]] = 0) { return x; } int main() { const int& x = foo(); // No error return x; } ```

[zygoloid]

This seems like an oversight to me. Might be as simple as stepping through CXXDefaultArgExprs at the right place.