Clangd support: pass in compiler flags for header files

[tomv564]

Clangd supports a non-standard meta field on didOpen.

LSP would have to extract the extra flags using per-project configuration:
https://github.com/Valloric/ycmd#ycm_extra_confpy-specification

[tomv564]

Considering compdb as alternative: Sarcasm/irony-mode#323

[3v1n0]

I think it's a good workaround for now, however the way of using compdb has changed (and not really much documented :)). You've to do something like:

compdb -c compdb.complementers=headerdb -p . update # or pointing the dir for the compile_commands.json
compdb -c compdb.complementers=headerdb -p . list > compile_commands_full.json
mv compile_commands_full.json compile_commands.json

The first command will generate an headerdb.json file, the second will merge the two in one (you might also act on multiple directories at once, by providing multiple -p <path>'s)...

[tomv564]

Having tested cquery with a compilation database in #208 I'm convinced that the clangd extension on didOpen is not worth it. I hope we can improve the generation / maintenance of compilation DBs with header files with a clangd/cquery-specific LSP package.