Add lang/cling: Interactive C++ Interpreter Based on LLVM and Clang Libs

Cling is an interactive C++ interpreter,
built on the top of LLVM and Clang libraries.
Its advantages over the standard interpreters are that it has command prompt
and uses just-in-time (JIT) compiler for compilation.
One of Cling's main goals is to provide contemporary, high-performance
alternative of the current C++ interpreter in the ROOT project "CINT".
The backward-compatibility with CINT is major priority during the development.

Its main advantages are:

Production-grade parser.
Just-in-time compiler (JIT).
Modular C++ API from the ground up.
Separate parser and execution engine.
C++ 11 support through clang.

PR: 215689
Submitted by: Mahdi Mokhtari
Reviewed by: mat, novel
Differential Revision: https://reviews.freebsd.org/D9012