Change the settings UI to show the actual default value, instead of empty

@CieroCeramics We thought this issue was a debugger bug, but according to @chrisant996 it sounds like a language service configuration issue that is resolved as @chrisant996 describes. Can we close this issue?

The defines property solved the issue for me. But I'm having to manually clean it up in every project, because each project had UNICODE injected into it. I can sort of understand that approach, except that since defines appeared to be empty I didn't know the UNICODE value was being stamped into every project.

@chrisant996 I don't fully understand what doc change would be best. You could file a doc issue at https://github.com/microsoft/vscode-docs/issues . The default defines are shown at https://code.visualstudio.com/docs/cpp/config-msvc#_cc-configurations .

What could have made things much more straightforward and understandable would have been if the Settings UI showed the actual default value, instead of empty.

1. It's non-intuitive that I have to use a different UI than Settings, to change the settings.
2. Very early on, I found the C_Cpp.default.defines section. But it was empty. So that makes it looks as though that's not where the "UNICODE" define was coming from.
3. Having -- or not having -- a "UNICODE" define is such a fundamental setting for a Windows program that I think it would make sense for the FAQ and core cpptools configuration documentation to specifically use it as an example. Especially since there is a hidden implicit hard-coded default if defines isn't set in any of the many places it could be set.
4. Ideally the Settings UI should show the real default value of defines, instead of empty.

Originally posted by @chrisant996 in https://github.com/microsoft/vscode-cpptools/issues/5244#issuecomment-764445169

Start with the C_Cpp.default.defines configuration and the Settings UI behavior described in the issue, then review the linked C/C++ configuration documentation for the documented defaults. Determine how the UI currently represents an unset defines value and verify that it displays the actual default without obscuring project-level overrides.