... instead of manual filtering
Some -Wwarning-type flags were removed because in combination with
-Werror they caused clang to fail when the warning type was unknown.
Rather than enumerating them (a manual process with the extra risk of
leaving warnings disabled longer than necessary), this adds
`-Wno-unknown-arning-option` which disables the warnings (that are
becoming erors through -Werror) raised when a warning's name is unknown.
By running make compile-commands a `compile_commands.json` in the RIOT base
directory. With the environment variable `COMPILE_COMMANDS` the path of
this file can be changed to a custom location.
The `compile_commands.json` will contain the exact compile command, but
as additional flag `-I/usr/$(TARGET)/include` is added to work around
`clangd` not being able to locate the newlib system headers. The
additional includes can be overwritten using the environment variable
`COMPILE_COMMANDS_EXTRA_INCLUDES`.
