[clang] [CIR] Upstream CastOp and scalar conversions (PR #130690)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

We have to be mroe careful I think with uses of the `errorNYI` in a few places, we are using it too much like `assert` when it is more like `emitDiag`. 

Also, this patch uses `auto` a bunch in ways that are contrary to the coding standard.  See : https://llvm.org/docs/CodingStandards.html#use-auto-type-deduction-to-make-code-more-readable

`auto` is really only to be used (rule of thumb) when the type is already 'listed' on the RHS, or naming the type is so obnoxious as to be irrelevant (iterators/etc).

https://github.com/llvm/llvm-project/pull/130690