[clangd] Use `ObjCProtocolLoc` for generalized ObjC protocol support

This removes clangd's existing workaround in favor of proper support
via the newly added `ObjCProtocolLoc`. This improves support by
allowing clangd to properly identify which protocol is selected
now that `ObjCProtocolLoc` gets its own ASTNode.

Differential Revision: https://reviews.llvm.org/D119366