Skip installation checks for clangd server if it has already been set up locally.

[EugeneSqr]

In Arch Linux clangd comes as a part of the clang package.

After successful installation the binaries are placed into /usr/bin like this:

lrwxrwxrwx 1 root root       8 Oct  3 19:11 clang -> clang-18
-rwxr-xr-x 1 root root    143K Oct  3 19:11 clang-18
-rwxr-xr-x 1 root root     25M Oct  3 19:11 clangd
-rwxr-xr-x 1 root root     99K Oct  3 19:11 clang-format
...

which makes clangd readily available:

$ clangd --version
clangd version 18.1.8
Features: linux
Platform: x86_64-pc-linux-gnu

Currently the clangd LSP server installation script expects local clangd binaries to be named like "clangd-X", which isn't always the case as shown above. This aborts LSP server installation with the following message "Could not find an installable clangd release!"

This PR bypasses the check when clangd is set up allowing LSP server installation to complete successfully.

[mattn]

Thank you