transpile: Compute and store override_tys ahead of time

[Rua]

• Depends on transpile: Reduce repetition in simd function handling #1757
• Depends on transpile: Minor improvements of cast handling #1756
• Fixes transpile: const macros aren't translated with portable types #1321

Before proceeding to remove override_ty, I added a temporary assert to compare the value passed to convert_expr with the value being generated by the new method. The assert eventually passed in the full CI suite, which likely means they match perfectly.

There was a comment left here concerning this issue:

c2rust/c2rust-transpile/src/translator/mod.rs

Line 4198 in 4887dce

// TODO We'd like to get rid of this cast eventually (see #1321).

I don't think the cast can be removed, though. In the snapshot test, the LITERAL_FLOAT and NESTED_FLOAT macros switched type from double to float, which removed some casts to float, but caused new casts to double to be added elsewhere.

[Rua]

This can now be reviewed. It's a big change, certainly. But since it fixes a major pain point with macro expansion, maybe worth it?