swift234

[CodeCompletion] Avoid choking on a doubly-typechecked expr

First, fix a case of type checking an expression twice, which is against
the design of the type checker. We hit this because we can type check
the "context" in
  let x = foo.#^COMPLETE^#
and then type check the "parsed expression", which is contained in the
context here.

Second, make the hack from rdar://20738314 more robust so that if we
*do* double typecheck for some reason we won't just choke on an
AutoClosureExpr.  I filed rdar://21466394 to audit for other cases of
double typechecking and remove this hack.

Fixes rdar://21346928

Swift SVN r29527