Completion of anonymous structures and unions inside designated initializers

[aganm]

(this issue is a continuation of #142)

Completion of nested initializers now works as of clangd version 13.0.0.
(Debian clangd version 13.0.0-++20210619101720+e1adf90826a5-1~exp1)

Unfortunately, it does not seem to support anonymous structures and unions inside designated initializers.

// main.c

struct test {
	struct {
		int x;
		struct {
			int y;
		} nested;
	} nested;

	struct {
		int z,w;
	}; // anonymous struct
};

int main()
{
	struct test t = {
		.nested = { .x = 3, .nested = { .y = 6 }, }, // full completion for named nested members
		. // no completion for anonymous struct members z,w in designated initializer
	};

 	return 0;
}