[QoI] Improve diagnostics when accessing .init on a non-metatype

[jtbandes]

Resolves SR-3236 by falling through to existing logic when all candidates are inaccessible. Maybe not perfect, but better than the current state of affairs I think.

[jtbandes]

@swift-ci please smoke test macOS

[jtbandes]

@swift-ci please smoke test

[jtbandes]

cc @xedin @rudkx @jrose-apple who were involved with #5237.

[xedin]

LGTM!

Edit (since I've only read the code and not comments):
I think it's a bit out of context with title, since it checks for acessibility instead of base type, so I think to resolve SR-3236 we need a better diagnostic as @rintaro suggested...

[jrose-apple]

I agree with @xedin. This shouldn't be an access control test; we should just reject this member access entirely.

[jrose-apple]

(and the diagnostic shouldn't state anything about "access" or "inaccessible")

[jtbandes]

The main point of this PR was to avoid spewing a candidate list when some candidates are actually accessible, but wrong for other, diagnosable reasons (e.g. being a type member while the call site is trying to access an instance member).