Indicate inactive compilation condition code branches

[award999]

Is your feature request related to a problem? Please describe.
Add a setting to show some kind of indication for inactive compilation condition branches.
Like Xcode has had for a while now.

Xcode simply grays out the inactive branch:

Describe alternatives you've considered
Eyeballing inactive branches.

Additional context
Add any other context or screenshots about the feature request here.

[award999]

Asked by user of swift extension swiftlang/vscode-swift#1733