Please consider adding a slot operator on sys-devel/llvm dependency. Mesa need to be rebuilt after libLLVM soname changes.
Indeed when i updated llvm from version 3.3-r3 to 3.4.2 mesa rebuild was done by @preserved-rebuild because several mesa bins were linked to libLLVM-3.3.so. Done in x11 overlay for mesa-9999.
Fixed in -10.2.7. Thanks!
In 10.2.7/8, 9999, and 10.3.0 which I'm committing now. Marking fixed.