[clangd] CMake: express -Iclangd/ at top level and inherit

For files directly under clangd/, -Iclang-tools-extra/clangd (and the
equivalent for generated files) are not required, as CMake/the compiler puts
these directories on the include path by default.

However this means each subdirectory needs to
include_directories(.. ${CMAKE_CURRENT_BINARY_DIR}/..) etc, and this
proved annoying and error-prone to maintain and debug.

Since include_directories is inherited by subdirectories, we just
configure this explicitly at the top level instead.