c++: Add diagnostic when operator= is used as truth cond [PR25689]

c++: Add diagnostic when operator= is used as truth cond [PR25689]

When compiling the following code with g++ -Wparentheses, GCC does not
warn on the if statement. For example, there is no warning for this code:

struct A {
	A& operator=(int);
	operator bool();
};

void f(A a) {
	if (a = 0); // no warning
}

This is because a = 0 is a call to operator=, which GCC does not handle.

This patch fixes this issue by handling calls to operator= when deciding
to warn.

Bootstrapped and regression tested on x86_64-pc-linux-gnu.

	PR c++/25689

gcc/cp/ChangeLog:

	* call.cc (extract_call_expr): Return a NULL_TREE on failure
	instead of asserting.
	(build_new_method_call): Suppress -Wparentheses diagnostic for
	MODIFY_EXPR.
	* semantics.cc (is_assignment_op_expr_p): Add function to check
	if an expression is a call to an op= operator expression.
	(maybe_convert_cond): Handle the case of a op= operator expression
	for the -Wparentheses diagnostic.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wparentheses-31.C: New test.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>