llvm.eclass: An eclass to handle dependencies on slotted LLVM

llvm.eclass: An eclass to handle dependencies on slotted LLVM

Introduce an eclass to support dependencies building against slotted
LLVM.  It provides a function to find the newest installed LLVM version
that is not newer than the max supported slot, and a trivial pkg_setup()
implementation that adds executable directory of this install to PATH.
This ensures that:

a) build systems will find the correct llvm-config and use it to use
   the correct version of LLVM,

b) CMake's find_package() will find the correct LLVMConfig / ClangConfig
   files.