Problem:
Some built-in ftplugins set omnifunc/tagfunc/formatexpr which causes
lsp.lua:set_defaults() to skip setup of defaults for those filetypes.
For example the C++ ftplugin has:
omnifunc=ccomplete#Complete
Last set from /usr/share/nvim/runtime/ftplugin/c.vim line 30
so the changes done in #95c65a6b221fe6e1cf91e8322e7d7571dc511a71
will always be skipped for C++ files.
Solution:
Overwrite omnifunc/tagfunc/formatexpr options that were set by stock
ftplugin.
Fixes#21001