Fix address of function in decltype context

[DutChen18]

There is a check that tries to prevent you from taking the address of a wasm function from genericjs, or vice versa. The check uses CurScope->getFnParent() to get the function in which the address of operator takes place. This is not a reliable way to get the current function declaration, and its usage here allows some invalid code to sneak past the check.

#include <cheerp/clientlib.h>

struct functor {
	[[cheerp::wasm]]
	void wasm_func() {}
};

template<class T>
[[cheerp::genericjs]]
auto js_func() -> void(functor::*)() {
	return &T::wasm_func; // address of wasm function here
}

[[cheerp::genericjs]]
int main() {
	client::console.log(js_func<functor>());
}

getCurFunctionDecl() is a more reliable way to get the current function declaration, and using it, the above code no longer compiles (yay!).

There is one place in cheerp/client.h where the address of a wasm function is taken from inside a decltype expression in the return type of a genericjs function. Because the operand of a decltype expression is never evaluated, we can always allow taking the address of a function like this as long as it happens in a decltype expression. And, this way, there is no issue in cheerp/client.h.

[yuri91]

nice double fix!