Provide hint for auto declaration in Cpp2 code

[ntrel]

AFAIK, no Cpp2 declaration starts with an auto keyword, so forbid it.
Like the hints for namespace/class keywords.

Without this:

f: () = {
    ...
    auto x = 1; // causes ill-formed initializer error but line is first line of `f`, not this line
    auto [x,y] = (1,2); // passes cppfront but errors in C++1
}

With this, both auto declarations are caught with correct line numbers:

test.cpp2(8,10): error: unknown declaration (use `name: type = initializer` instead?)
test.cpp2(9,10): error: unknown declaration (use `name: type = initializer` instead?)

BTW I have emailed Herb requesting a CLA form.

[hsutter]

Thanks, this looks good! CLA is on the way.

Could you please update it to follow the style in the immediately following check for namespace? That is, use the next token (if there is one) in the error message, and make the error message follow that pattern... something like "to define an object " + name + ", write '" + name + " : /*type*/ = /*initializer*/;', or '" + name + " : /*type*/;' and initialize the object later". 🙏

[ntrel]

Now done, it prints:

test.cpp2(8,10): error: to define a variable x of type T, write 'x: T = /* initializer */'
test.cpp2(9,10): error: to define a variable v of type T, write 'v: T = /* initializer */'

The simple case parses the identifier, the auto [x, y] case just defaults to v.

[hsutter]

Looks good, thanks!