swift234

Cache results of visible decl lookup at top-level in a TranslationUnit.

This is a tradeoff, assuming that lookup in a particular module is less
likely to be repeated than lookup within the current source file. By
storing the cached results with the TU's own local lookup cache, the
results get cleared out when we parse additional decls.

Since this is only used by code completion, there's no performance benefit
for the compiler, or even for swift-ide-test...it's only in repeated lookups
that this will be useful.

Swift SVN r7169