AstGen: pre-scan all decls in a namespace

Also:
 * improve the "ambiguous reference" error by swapping the order of
   "declared here" and "also declared here" notes.
 * improve the "not accessible from inner function" error:
   - point out that it has to do with the thing being mutable
   - eliminate the incorrect association with it being a function
   - note where it crosses a namespace boundary
 * struct field types are evaluated in a context that has the struct
   namespace visible. Likewise with align expressions, linksection
   expressions, enum tag values, and union/enum tag argument
   expressions.

Closes #9194
Closes #9622

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