Introduce a centralized routine in Sema for diagnosing failed lookups (when

used as expressions).  In dependent contexts, try to recover by doing a lookup
in previously-dependent base classes.  We get better diagnostics out, but    
unfortunately the recovery fails:  we need to turn it into a method call  
expression, not a bare call expression.  Thus this is still a WIP.

llvm-svn: 91525