Enable ROOT's builtin_llvm=Off

This patch allows ROOT to be built against compatible external llvm (5.0
or 5.0.1). Note that we still need to build clang (eg. we require
builtin_clang=On) due to the ROOT-specific patches which are not yet
upstream.

Since we have externally installed llvm, we configure and build clang as
a standalone project. The configuration relies on finding llvm-config-5.0
and uses an adapted version of the standard clang standalone build
procedure.

Clang provides dependencies such as FileCheck and not which are used by
cling's testsuite and are not being installed with the standard llvm
package.

Cling (which depends on llvm and clang) is built as a clang tool to avoid
unresolved dependencies to clang and complicating further the already
complicated cmake setup.

This patch intends a minimal change and follows the initial (suboptimal)
design to configure and build llvm, clang and cling as part of ROOT. An
ultimate solution would be to have llvm, clang and cling built as separate
standalone projects (following the recommended way by the LLVM cmake
developers).