This replaces the obscure, inefficient lookup into extensions with
something more straightforward: walk all of the known extensions
(available as a simple list), then eliminate any declarations that
have been shadowed by other declarations. The shadowing rules still
need to consider the module re-export DAG, but we'll leave that for
later.
As part of this, keep track of the last time we loaded extensions for
a given nominal type. If the list of extensions is out-of-date with
respect to the global generation count (which tracks resolved module
imports), ask the modules to load any additional extensions. Only the
Clang module importer can currently load extensions in this manner.
Swift SVN r5223
The writer stores the paths to the .swift files, the reader just loads
those .swift files. Fun!
No version validation yet. No separate ModuleReader class yet. No use of
the SerializedModule architecture yet, except for creating a dummy module
when the .sm file is corrupted.
Swift SVN r5164
