Add a new interface GetMethodWithArgs() to handle method lookup
by name and example argument list.

Add a new interface Init(clang::Decl*) to allow resetting from
with a known class declaration.

Mark the places in function lookup where we should do something
special for constructors.

Make the other methods which do a function lookup go through
GetMethod() so that we have a single place where this work is
done.


git-svn-id: https://root.cern.ch/svn/root/trunk@48416 27541ba8-7e3a-0410-8455-c3a389f83636