swift234

[serialization] Lazily load top-level decls, extensions, and operators.

This switches from simple lists of decls to name-based on-disk hash tables,
which allows decls to be loaded lazily when doing simple lookup (but not
code completion, at least not yet).

The on-disk hash table implementation is borrowed from Clang; eventually
it will be pushed down to LLVM's Support library. (Fortunately the
implementation is header-only.)

This breaks a few tests that rely on magic protocols like
IntegerLiteralConvertible, because the type checker won't have seen the
types that conform to those protocols yet. This will be fixed by doing
an additional "hey, modules, got any of these?" lookup.

Swift SVN r7259