Switch LLVM reflection utilities to be CodeInstance based

[Keno]

This is a natural continuation of the stream of work begun in #53219. In particular, with #56555, it is important to be able to pass a CodeInstance, since the ABI may be overwritten and without knowing that, codegen will generated incorrect code for the CodeInstance.

[vtjnash]

The src argument is not optional (nullable)

[Keno]

The src argument is not optional (nullable)

It is in this PR.

[vtjnash]

Then this PR cannot be merged, since it cannot be optional, as codegen has no other reliable way to get that

[Keno]

It just picks out the .inferred field and errors if its not there. The use case is for reflecting on CodeInstances that have the field. If they don't have the field, it obviously needs to be passed in separately.