[clangd] Heuristic to avoid desync if editors are confused about newline-at-eof

As strange as it seems to our files-are-strings view of the world, some editors
that treat files as arrays of lines can get confused about whether the last line
has a newline or not.

The consequences of failing to handle a bad incremental update are catastrophic.
If an update would be valid except for a missing newline at end of file, pretend
one exists.

This fixes problems still present in neovim where deleting all text often leads
to a desync shortly afterwards: https://github.com/neovim/neovim/issues/17085

Differential Revision: https://reviews.llvm.org/D135508