[BUG] can not override a virtual function to pure virtual

[Xeverous]

Describe the bug
cppfront does not allow to override a function into pure virtual. Whether base type function is pure or not has no impact.

To Reproduce
sample code:

base: type =
{
    d: (virtual move this);
    f: (virtual this) -> int = 1;
}

derived: type =
{
    this: base = ();
    f: (override this) -> int;
}

expected result - something like this:

class base
{
public:
    virtual ~base() = default;
    virtual int f() { return 1; } 
};

class derived : public base
{
public:
    int f() override = 0;
};

actual result: main.cpp2(10,5): error: a function must have a body ('=' initializer), unless it is virtual (has a 'virtual this' parameter) or is defaultable (operator== or operator<=>)

Command lines

cppfront main.cpp2 -p, built from commit dc3758a

Additional context

Conflicting documentation - https://hsutter.github.io/cppfront/cpp2/types/ says:

A pure virtual function is a function with a virtual this or override this parameter and no body.

[gregmarr]

Probably need to add a check for override here.

cppfront/source/sema.h

Lines 1636 to 1649 in dc3758a

	// A nonvirtual and nondefaultable function must have an initializer
	if (
	n.is_function()
	&& !n.is_virtual_function()
	&& !n.is_defaultable_function()
	&& !n.has_initializer()
	)
	{
	handle_error(
	n.position(),
	"a function must have a body ('=' initializer), unless it is virtual (has a 'virtual this' parameter) or is defaultable (operator== or operator<=>)"
	);
	return false;
	}

I wasn't even aware that this was a thing. I've never seen a pure virtual override, but I can see where it can be useful.