Rust: Fix common FPs for rust/unused-variable and rust/unused-value.

rust/ql/src/queries/unusedentities/UnusedVariable.qll

@@ -43,4 +43,8 @@ predicate isAllowableUnused(Variable v) {
   or
   // a 'self' variable
   v.getText() = "self"
+  or
+  // a common source of false positives is match arms that are misrecognized as
+  // a variable, having not been correctly resolved
+  v.getPat().getParentNode() instanceof MatchArm
 }