ApplicativeDo: Set pattern location before running exhaustiveness checker

This improves the error messages of the exhaustiveness checker when checking statements which have been moved around with ApplicativeDo.

Before:

Test.hs:2:3: warning: [GHC-62161] [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding:
        Patterns of type ‘Maybe ()’ not matched: Nothing
  |
2 |   let x = ()
  |   ^^^^^^^^^^

After:

Test.hs:4:3: warning: [GHC-62161] [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding:
        Patterns of type ‘Maybe ()’ not matched: Nothing
  |
4 |   ~(Just res1) <- seq x (pure $ Nothing @())
  |

Fixes #22483 (closed)