r174943 - Fix a bug reduced from a crash when trying to use modules with libc++. We check

[Rafael Espíndola]

> We can't call getCachedLinkage on both functions in isSameEntity. We only
> know one of them has cached linkage, not both.
>
> IMO, the right way to fix this is to get rid of the linkage cache, and
> compute and store linkage when an entity is first declared. We only need
> caching and recomputation for visibility.

I agree. I actually noticed the assert was missing while rebasing my
old branch with no caching. I will keep it locally during development.

Thanks,
Rafael