[clangd] Add heuristic for dropping snippet when completing member function pointer

This implements the 1st heuristic mentioned in https://github.com/clangd/clangd/issues/968#issuecomment-1002242704:

When completing a function that names a non-static member of a class, and we are not inside that class's scope, assume the reference will not be a call (and thus don't add the snippetSuffix)

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D137040