[Diagnostics] Make sure that fixes associated with function builders are found and diagnosed
Current logic in `applySolutionFixes` didn't actually look inside
of single-expression closures (because of brace statement) or
closures which are transformed by function builders.
Resolves: rdar://problem/51167632
(cherry picked from commit d8debe7a5717742ccce9a770c9aef78fc97e1c63)