swift234

Added ExternalNameLookup, a mechanism that allows
UnqualifiedLookup to ask an external source for
names.  There are two phases to this external lookup:

- Before consulting globals in other modules,
  UnqualifiedLookup calls lookupOverrides() to see
  if there are any results that should override the
  results from modules.  (N.b.: these should not be
  able to override names that are locally defined.)

- After consulting globals in other modules,
  UnqualifiedLookup as a last resort calls
  lookupFallbacks() to see if there's anything out
  there at all that could serve that name.  This may
  be more computationally expensive.

These hooks are used by LLDB's expression parser to
resolve names for persistent variables (akin to the
existing $0, $1, ... variables) and variables local
to the current frame.


Swift SVN r9014