This starts propagating is_extern on import, and warns when merging an
imported non-extern declaration with a local non-extern declaration.
Note this doesn't address import conflicts yet (i.e., two libraries
define an equivalent name) because they don't call merge logic.
On merge, I'm only setting values when new_is_definition because I think
it better matches the comment and resulting behavior. Note I now set
them even for bad redefinitions; I think this matches the comment, and
there's not a perfect choice here. I could change the flow back if
preferred.
Note this puts a spotlight on invalid nodes on imported decls, which I
think I'm going to need to address now. This isn't addressed by
ImportRef logic directly because the Function's decl_id is a
FunctionDecl, rather than the ImportRef that led to it. While I could
add the ImportRef link to each decl, I think adjusting the associated
NodeId is a better approach.
The purpose of the newline is to make it clearer where a given
diagnostic begins and ends, particularly as the first message of a
diagnostic may not be the error.
This is a trivial code change, but ripples edits through test files.
For now, a builtin function is defined by specifying a string literal
initializer in a function declaration:
```carbon
fn MyBuiltin(a: i32) -> i32 = "builtin.name";
```
End-to-end support is included for a sample `"int.add"` builtin
performing integer addition, covering constant evaluation and code
generation.
The implementation here needs substantial refactoring before we'll be
ready to start adding more builtins. That refactoring work will be
coming next. This change is aiming to checkpoint some incremental
progress.
Jon Ross-Perkins
Richard Smith