* 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
f30aa25cbf