[libclc] Allow building with pre-built tools

Building the libclc project in-tree with debug tools can be very slow.
This commit adds an option for a user to specify a dierctory from which
to import (e.g., release-built) tools. All tools required by the project
must be imported from the same location, for simplicity.

Original patch downstream authored by @jchlanda.

GitOrigin-RevId: 0aeeff3059e79b86f55ed92a4488bdee8fa66e12
1 file changed