Fix LocalVector<T>::clear when T has no default constructor

[HolonProduction]

What problem(s) does this PR solve?

This code did previously not compile:

struct Test {
	int i;
	Test() = delete;
	explicit Test(int p_i) : i(p_i) {}
};

LocalVector<Test> t;
t.push_back(Test(1));
t.clear();

because LocalVector::clear did previously use resize internally, it required T to have a default constructor.

[Ivorforce]

Strange problem to have but I agree this makes sense.

[lawnjelly]

First I was like "huh, won't it also fail with resize(0)"?

And it will, but I guess this is for the pattern of pushing back initialized values, and clear shouldn't require a default constructor logically.

The problem with resize() is that the compiler sees it touches the default constructor.

So I also agree with the PR (even if it's a little strange having to do this).

[HolonProduction]

huh, won't it also fail with resize(0)

Yeah if some operations fail that's fine, the issue is primarily that the local vector destructor uses clear. So effectively it makes LocalVector entirely useless if you have no default constructor (or a private one). In practice this just isn't common because most classes need a default constructor for serialization.

[Repiteo]

Thanks!