* stage2 AstGen: add missing compile error for declaring a local
   that shadows a primitive. Even with `@""` syntax, it may not have
   the same name as a primitive.
 * stage2 AstGen: add a compile error for a global declaration
   whose name matches a primitive. However it is allowed when using
   `@""` syntax.
 * stage1: delete all "declaration shadows primitive" compile errors
   because they are now handled by stage2 AstGen.
 * stage1/stage2 AstGen: notice when using `@""` syntax and:
   - treat `_` as a regular identifier
   - skip checking if an identifire is a primitive

Check the new test cases for clarifications on semantics.

closes #6062