Wrap a recurring question into an interface: getPointerToGlobalFromJIT().

(cherry picked from commit 0b7973cecb915abd7e0add8b907cec6784a30313)
Interpreter::compileFunction(): compile this function and give me its address.

(cherry picked from commit 04de3a90cddb2718dc263ee104854b5395bfd869)
Fix for namespaces; simplify; use Interpreter::compileFunction().

(cherry picked from commit 9a9e0a04822c237b1da303688293c2600bd53161)
Functions are extern "C".

(cherry picked from commit c7caadda0902a0b5cf7194c584ffd3cc328827a1)
Move function compilation into cling::Interpreter.