Properly handle declarations at local scope

[mohammadfawaz]

For example, the following code results in an ICE:

script

fn foo() {
    struct S {}
}

ICE:

4 |     struct S {}
  |     ^^^^^^^^^^^ Internal compiler error: Unexpected struct declaration found.
Please file an issue on the repository and include the code that triggered this error.

Most declarations exhibit the same behavior, such as enums, storage decl, type aliases, etc.

At the very least, we should error out properly instead of throwing an ICE.

[otrho]

Wow. How did this stay a thing for so long?

[vaivaswatha]

This isn't really an ICE, but an error that prints an ICE like message.

[mohammadfawaz]

Hahaha... Toby added this UnexpectedDeclaration error which indeed should be an internal compiler error because we should be catching these issues much much earlier.