Rust: Rearrange the unused variable query logic so that it's clearer what the comments apply to.

Author: geoffw0

rust/ql/src/queries/unusedentities/UnusedValue.ql

@@ -16,11 +16,11 @@ import UnusedVariable
 from AstNode write, Ssa::Variable v
 where
   variableWrite(write, v) and
+  not v instanceof DiscardVariable and
+  not write.isInMacroExpansion() and
+  not isAllowableUnused(v) and
   // SSA definitions are only created for live writes
   not write = any(Ssa::WriteDefinition def).getWriteAccess().getAstNode() and
   // avoid overlap with the unused variable query
-  not isUnused(v) and
-  not v instanceof DiscardVariable and
-  not write.isInMacroExpansion() and
-  not isAllowableUnused(v)
+  not isUnused(v)
 select write, "Variable $@ is assigned a value that is never used.", v, v.getText()