ClangLanguageService should decide whether to withhold fallback diagnostics based on previous build settings not upcoming build settings

[ahoppen]

ClangLanguageService.publishDiagnostics gets the build settings for the file to determine whether it should withhold them because they are fallback settings. But really, what matters here is whether it had build settings at the point when the last update was sent, not whether build settings can be retrieved after the clangd sent a publish diagnostics notification. To do this, we probably need to keep track of which files have proper build settings and which ones have fallback build settings in ClangLanguageService.

[ahoppen]

Synced to Apple’s issue tracker as rdar://138006475