[modules] Cache 'acceptable decl' lookups for namespaces. In projects with
thousands of modules, each of which declares the same namespace, linearly scanning the redecl chain looking for a visible declaration (once for each leaf module, for each use) performs very poorly. Namespace visibility can only decrease when we leave a module during a module build step, and we never care *which* visible declaration of a namespace we find, so we can cache this very effectively. This results in a 35x speedup on one of our internal build steps (2m -> 3.5s), but is hard to unit test because it requires a very large number of modules. Ideas for a test appreciated! No functionality change intended other than the speedup. llvm-svn: 261161
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