Add clang rules for RS_LLVM_* tools.

This change adds new variables for RS_LLVM_* tools, which correspond
to the version of LLVM checked in currently to external/, instead of
mapping to the latest host prebuilts. This will help prevent version
differences for newer IR generated by the updated host prebuilts, which
then might not be readable with the on-device LLVM bits we build from
external/.

Change-Id: I6c7b0fa9082e9c8244cc508cb23960569a057740