Add fallback case when the function is found in the AST but code wasn't generated
for it yet. In that case we can force cling to generate the code for that function
by wrapping the AST node in a transaction and asking the interpreter to produce
code for it.


git-svn-id: https://root.cern.ch/svn/root/trunk@48421 27541ba8-7e3a-0410-8455-c3a389f83636