Show error or warning message when compilation database could not found by clangd

[ghentschke]

As it is possible to open any file in the new LSP-based C/C++ editor, it is not guaranteed that the parent project contains a compilation database (e.g. compile_commands.json). Since much functionality of clangd relies on the availability of such a database, it could not work properly (e.g. used includes could not be found by clangd).

IMO it makes sense to inform the user that the parent project lacks of a compilation database.