[clang][Sema] Add CodeCompletionContext::CCC_ObjCClassForwardDecl

[dgoldman]

Summary

• Use this new context in Sema to limit completions to seen ObjC class names

• Use this new context in clangd to disable include insertions when completing ObjC forward decls

[kadircet]

clang-tools-extra/clangd/CodeComplete.cpp

418

rather than doing this here, can you pass ContextKind into headerToInsertIfAllowed ? we already have some logic there that does something similar (e.g. check if there's a decl in the main file). it'd be nice to group them together, in case we want to extend this in the future. (it'd be great if we can just have a contextAllowsHeaderInsertion helper, which can only return true for ObjCClassForwardDecl for now).

[dgoldman]

Add a contextAllowsHeaderInsertion helper to clangd

[dgoldman]

Rebase

[kadircet]

thanks