Remove GetOptionsDiagnostics

[jakebailey]

"Options diagnostics" were just config file diagnostics + global diagnostics.

This was odd, because we then took that in places and combined it with... config file diags and global diags. And then not deduping it, sometimes.

This PR just removes GetOptionsDiagnostics altogether, instead leaving behind just GetConfigFileParsingDiagnostics and GetGlobalDiagnostics.

The language server still does not ever call GetGlobalDiagnostics. I am not sure where such a thing would go, since global diagnostics are generated as a side effect of doing checking (e.g., when we figure out that AsyncIterable was never declared or something).

There's a baseline change, but I think it's correct and positive.