FindLLVM: allow LLVM/Clang 3.6
FindLLVM: allow LLVM/Clang 3.6

As beignet now works with LLVM/Clang 3.6, accept this version
when searching for llvm-config.

Signed-off-by: Rebecca Palmer <2decb937e7eca5e941ec22bc308fd452fe4ab2bb@zoho.com>
Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,zhenyw/beignet,zhenyw/beignet,zhenyw/beignet,wdv4758h/beignet