Run the VarDeclUsageChecker on top level code declarations.

[KingOfBrian]

Run the VarDeclUsageChecker on top level code declarations so unused variables correctly generate warnings.

Resolves SR-2115.

[KingOfBrian]

@swift-ci please test

[slavapestov]

Some more tests would be nice -- cover the other things that the VarDeclUsageChecker can diagnose

[KingOfBrian]

Good eye @slavapestov -- the additional tests definitely showed some issues. I had one misconception about what a TLCD was, and also didn't realize that the method was being pumped and would execute with partial state. I changed the code to run only on the last pump, and to walk all of the TLCD with one VarDeclUsageChecker.

[KingOfBrian]

Wow, should of ran the test suite locally! This fix uncovered 70 unexpected failures in the unit tests. They all seem like valid warnings, but it's going to take a few to comb through them all. I'm going to convert to let whenever suggested, but add an empty array to use all the variables instead of replacing with _ since that may alter some of the test behavior.

[KingOfBrian]

So I realized I need to be smarter with the top level unused-var declaration checks -- this will still incorrectly warn on cross-file globals. I'm going to close this for now so I can make sure I understand everything. Thanks!