package/libclc: switch to use the frozen, legacy mirror

package/libclc: switch to use the frozen, legacy mirror

The LLVM project has switched to using a monorepo to host all their
components. The separate, individual repositories have been closed
late 2020 / early 2021. The libclc repository is no longer.

Switch to using the libclc source from the llvm legacy and frozen
mirror.

Even though we could switch over to using the github helper, we just
keep using the git download method: it is a small repository, and it
will not impact people that were already using it.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Valentin Korenblit <valentinkorenblit@gmail.com>
Cc: Michael Opdenacker <michael.opdenacker@bootlin.com>
Acked-by: Romain Naour <romain.naour@gmail.com>
---
Changes v1 -> v2:
  - keep everything as-is, just switch to the frozen mirror