Rust: Fix some false positives for rust/unused-variable and rust/unused-value

[geoffw0]

Fix some common false positive results for the rust/unused-variable and rust/unused-value queries. Typically these results look something like this:

match foo {
  MY_CONSTANT => { ... }
  ...
}

Due to incorrect code, incorrect extraction, and/or extraction with the wrong settings MY_CONSTANT is misrecognized as a variable and the query observes that it is not used. I've been unable to produce a test case but its common in the wild and I expect DCA to produce many fixed examples of this.

[geoffw0]

Unfortunately it turns out this change does exclude some correct results (see the test). I'm not sure how I can improve that, short of looking at the case of the variable name (i.e. heuristic). Lets wait for the DCA results and see what we've got.

[geoffw0]

DCA showed 1076 results removed. I've reviewed a large number of these and nearly all resemble the typical example in the PR comment above. We do lose a tiny number of results where the target really was a (used or unused) variable - I've since narrowed the change slightly to address that (by checking capitalization as well).