This is strange. When I run the command I get:
john@john-ThinkPad-T510:~$ sudo apt-get install libclang-dev
[sudo] password for john:
Reading package lists... Done
Building dependency tree
Reading state information... Done
libclang-dev is already the newest version (1:10.0-50~exp1).
libclang-dev set to manually installed.
The following package was automatically installed and is no longer required:
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 19 not upgraded.
This seems to suggest it is available.
Maybe try something like "sudo apt-get install libclang-dev.10"?
I don"t know what else to suggest except perhaps enquiring on an Ubuntu forum.
Sorry I cannot be of more help.