[compiler] Bad diagnostics when scalar initializer constraint is mixed

### Describe the bug

If you try to write a mixed constraint like `valueof string` over a scalar initializer parameter, you get _no diagnostics_ until that initializer is actually called. At the call site, the only error is about the value being unassignable to ErrorType, rather than anything to do with the invalid constraint.

Scalar initializers do not accept mixed constraints and behave as if all of their parameter constraints have `valueof` in front of them.

### Reproduction

https://typespec.io/playground/?e=%40typespec%2Fopenapi3&c=c2NhbGFyIFMgewogIGluaXQgaShzOiB2YWx1ZW9mIHN0cmluZyk7Cn0KCmNvbnN0IHM6IFMgPSBTLmkoImZvbyIpOw%3D%3D&v=type-graph&vs=%7B%22type-graph%3Apath%22%3A%22%24.%28global%29.scalars.S.constructors.i.parameters.0%22%7D&options=%7B%7D

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.