Avoid computing the linkage too early. Don't invalidate it.

Before this patch we would compute the linkage lazily and cache it. When the
AST was modified in ways that could change the value, we would invalidate the
cache.

That was fairly brittle, since any code could ask for the a linkage before
the correct value was available.

We should change the API to one where the linkage is computed explicitly and
trying to get it when it is not available asserts.

This patch is a first step in that direction. We still compute the linkage
lazily, but instead of invalidating a cache, we assert that the AST
modifications didn't change the result.

llvm-svn: 176999