Avoid computing the linkage instead of avoiding caching it.

This mostly reverts 178733, but keeps the tests.

I don't claim to understand how hidden sub modules work or when we need to see
them (is that documented?), but this has the same semantics and avoids adding
hasExternalLinkageUncached which has the same foot gun potential as the old
hasExternalLinkage.

Last but not least, not computing linkage when it is not needed is more
efficient.

llvm-svn: 178739