mesa,llvm,meson: Update llvm to 8.0.1 plus define and use LLVM version globally

mesa,llvm,meson: Update llvm to 8.0.1 plus define and use LLVM version globally

- Add missing dependency on libedit
- Define LLVMVERSION on the same lines as GCCVERSION and other tools
- Use LLVMVERSION in mesa and meson.bbclass to get llvm version instead of
  hardcoding it
- Use llvm patches unmodified from meta-clang, helps in keeping them in
  sync
- Define PREFERRED_VERSION for llvm, llvm-native, nativesdk-llvm

(From OE-Core rev: 3c08b638348abd543fc92baf56c28ca16ae6aac6)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>