Resolve 'extern' types during type layout generation if possible

[expipiplus1]

Closes #5994 Closes #6437

[expipiplus1]

/format

[slangbot]

🌈 Formatted, please merge the changes from this PR

[csyonghe]

I wonder what will happen if you have an ordinary struct type with a field whose type is extern, and then you query the type layout of the outer struct type.

Another thing is if I lookup and get type layout first from a single IModile, then lookup and the type layout from a linked program on the same struct with extern field, will I get the up to date layout? iIRC we cache type layout on the type itself inside a dictionary owned by the module or something, and if so we may return the wrong result.