Convert DiagnoseEmptyLookup to use correction callbacks.

No new unit tests yet as there is no behavioral change
(except for slightly more specific filtering in
Sema::ActOnStartOfLambdaDefinition). Tests will be added
as the code paths are traced in greater depth to determine
how to improve the results--there are at least one or two
known bugs that require those improvements. This commit
lays the groundwork for those changes.

llvm-svn: 148382