Sema: Don't ignore current declaration if it's not ambiguous

[akarpovskii]

This fixes #17872.

Previous logic was introduced in c17793b by @Vexu to address issue #12429. For the following piece of code there's no ambiguity and no reason to ignore the only declaration.

pub const Example = struct {
    usingnamespace struct {}; // works without this line

    pub fn foo() void {
        _ = Example.foo;  // error: struct 'main.Example' has no member named 'foo'
    }
};

pub fn main() !void {
    Example.foo();
}

[andrewrk]

I'm sorry, this will need to be reworked after some recent changes to the way usingnamespace works (there are conflicts). Also note the existence of #20663, which is proposed but not yet accepted or rejected.