Teach Sema::isDeclInScope to handle overload sets constructed from

functions that occur in multiple declaration contexts, e.g., because
some were found via using declarations. Now, isDeclInScope will build
a new overload set (when needed) containing only those declarations
that are actually in scope. This eliminates a problem found with
libstdc++'s <iostream>, where the presence of using 

In the longer term, I'd like to eliminate Sema::isDeclInScope in favor
of better handling of the RedeclarationOnly flag in the name-lookup
routines. That way, name lookup only returns the entities that matter,
rather than taking the current two-pass approach of producing too many
results and then filtering our the wrong results. It's not efficient,
and I'm sure that we aren't filtering everywhere we should be.

llvm-svn: 82954