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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148382 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed