Concurrent modification error while referring to invalid aliases

[Snektron]

The following crashes on the latest master (13259ac):

const A = B;

test "Crash" {
    _ = @typeInfo(@This()).Struct.decls;
}

with the message "concurrent modification" and "Panicked during a panic. Aborting.".
Note that the following results in the expected error message "use of undeclared identifier 'B'":

test "No crash" {
    const A = B;
    _ = @typeInfo(@This()).Struct.decls;
}

[andrewrk]

This is the c++ compiler's hash map iterator detecting that the hash map was modified while being iterated over, which would be undefined behavior.

[betawaffle]

This seems to happen reliably for me when getting @typeInfo for a C import.

[FireFox317]

With my PR #4435 this only crashes when doing _ = @typeInfo(@This()).Struct.decls[0];, actually indexing the decls slice forces it to be resolved and just doing .decls doesn't resolve it yet.

Edit: Nvm, this has nothing to do with my PR, since it is not in master yet XD