No Configure IntelliSense button/notification appears if compilerPath is set to "" in a different workspace folder

[sean-mcmanus]

The bug is that the compilerPath set in any workspace folder is incorrectly only tracked globally instead of per-workspace folder, so the code that checks whether to show the Configure IntelliSense button incorrectly thinks a folder that doesn't have compilerPath set to "" has it set like that, i.e. at https://github.com/microsoft/vscode-cpptools/blob/main/Extension/src/LanguageServer/client.ts#L2726