move all cling/clang/LLVM dependencies to libCling.

This new libCling library is loaded in TROOT::InitInterpreter().
By moving all LLVM dependencies to libCling the LLVM symbols are not
exported to other parts of the program and linking with libCore does
not force the ROOT version of LLVM on its users. Also libCling does
solve the LLVM globals initialization order to the time libCling gets
loaded.