[clangd] Look for resource-dir relative to detected compiler path as a fallback (#203332)
If the standard resource directory (which is searched for relative to the clangd
executable) does not exist, look for one relative to the detected compiler as a
fallback. This handles some packaging schemes where clangd and clang are
installed in different prefixes and the resource directory is only located in the
latter.
Also print an error message to the log if the fallback didn't find an existing
directory either.