Don't wrap an ErrorInst as a subpattern of another pattern

[danakj]

When a pattern is an error, avoid wrapping it as a subpattern in a RefParamPattern or VarPattern. This allows further error handling to observe the error occurred without having to unwrap it.

This avoids a crash when an invalid associated constant is written which has a VarPattern in it. The associated constant machinery expects an AssociatedConstantDecl but was getting a VarPattern with an ErrorInst inside. Now it receives an ErrorInst directly, which it is already looking for.

This choice means that var statements in an interface will always be diagnosed as being non-constant, or as being a constant with a var, so we don't need an extra diagnostic saying that var is not allowed in an interface, as this just leads to two diagnostics on the same thing.

This crash was found by a fuzzer.

[josh11b]

This is fine, but I wonder if we can (eventually) give the advice that var isn't going to work in an interface in the diagnostic, instead of something suggesting you just have to fix the binding.

[danakj]

When a pattern is an error, avoid wrapping it as a subpattern in a RefParamPattern or VarPattern. This allows further error handling to observe the error occurred without having to unwrap it.

Maybe we can reword the "found runtime binding pattern in associated constant declaration" error if it looks ahead/behind somehow and finds a var.