swift234

Don't crash when forward-referencing a local variable.

Since we type-check local variables in statement order in a function, but
resolve names on a whole-scope basis, we can end up with a name that refers
to a local variable before its declaration has been reached. If this happens,
the variable won't have a type.

Checking this correctly may require comparing the order of statements in a
syntactic scope, but for now, just emit an error message if a reference
ever resolves to a variable without a type. (Note that UnqualifiedLookup
will have already tried to validate the decl, so if it's a global we just
haven't seen yet there's not a problem.)

Unfortunately, we won't get this right for top-level variables in script
source files, since those are still stored at global context and can be
validated on demand. They're never uninitialized, but zero-initialization
may not be valid for all global kinds.

<rdar://problem/15912025>

Swift SVN r13003