Pull Sema::isAcceptableLookupResult into SemaLookup. Extract the criteria into

different functions and pick the function at lookup initialization time.
In theory we could actually divide the criteria functions into N different
functions for the N cases, but it's so not worth it.

Among other things, lets us invoke LookupQualifiedName without recomputing
IDNS info every time.

Do some refactoring in SemaDecl to avoid an awkward special case in LQN
that was only necessary for redeclaration testing for anonymous structs/unions ---
which could be done more efficiently with a scoped lookup anyway.

llvm-svn: 91676