bug#61679: 29.0.60; Eglot: staying out of trouble

[Augusto Stoffel]

Eglot modifies several Emacs features, and it seems to do so
independently of the server capabilities.  For instance, it registers a
capf and Eldoc functions even if the server does't provide completions
respectively "hover"/signature help.

In some cases (capf, xref) this seems innocuous, in others (eldoc) Eglot
is really overwriting a possibly useful function with a possibly useless
one.

Now, the above are somewhat silly examples, but the Flymake case is
quite problematic.  There exist good servers that don't provide
diagnostics, and Eglot still overwrites `flymake-diagnostic-functions'.
To make things worse, LSP doesn't provide a way to the server to advertise
whether or not it provides diagnostics.

I would suggest that Elgot initially should keep
flymake-diagnostic-functions unchanged, and only overwrite it when the
first diagnostic comes from the server.