Add code folding for "case" sections

I filed a feature request on VS at https://developercommunity.visualstudio.com/idea/1020490/c-intellisense-add-code-folding-for-case-regions.html . Our implementation is shared with VS.

_Originally posted by @sean-mcmanus in https://github.com/microsoft/vscode-cpptools/issues/407#issuecomment-624988706_