Add TODO.


git-svn-id: https://root.cern.ch/svn/root/trunk@48536 27541ba8-7e3a-0410-8455-c3a389f83636
Add support in TClingCallFunc for calling virtual functions: when we have a 
CallFunc representation of a member function, we have its concrete address, and
if there is derived class there is no way how to evaluate the vtable and call the
correct virtual.

In order to evaluate the member's vtable, we either have to calculate it (which
is platform/implementation dependent) or create a trampoline function which calls
the target function. Thus codegen will generate code for vtable evaluation. The
trampolines are created for functions that could be possibly virtual - non-static
member functions, declared as virutal. A trampoline looks like this:

void unique_name(Base* This, MyClass2* a, MyClass3* b, double c, MyClass* ret) {
  if (ret) 
    *ret = This->function(*a, *b, c);
  else 
    This->function(*a, *b, c);
}

where we pass in explicitly the address that CallFunc is given (This), followed
by set of arguments (possibly needing conversion) and if the target is non-void
a last argument that is used to provide a storage for the return result.

The given arguments and the required by *LLVM* IR may differ - in that case 
simple argument conversions are done. For now primary pointers and builtins, which
covers pretty much entire LLVM type system.

Note that the first implementation is very rough and suboptimal, however it is 
very good starting point (milestone):
* Common code is extracted out in its separate routines (here a lot of refactoring more
could be done).
* cling::Value now stores besides clang::QualType, describing (soft-of inaccurately)
the llvm::GenericValue, LLVM-type which is the actual description of the generic value.
* cling::Value naming conventions improved;
* cling::Interpreter::getLLVMType is added temporarily and soon will be removed.

* TClingCallFunc now preallocates space for the this ptr and the return result.
(instead of multiple array copies)
* TClingCallFunc now stores not llvm::GenericValues but cling::StoredValueRefs
because the type conversions need llvm::GenericValue's description (which 
essentially is the type).
* Does better default argument evaluation: it could be even *more betterer* if
we query Sema to fold for us the constant expressions instead of decompiling them
to source and using cling for evaluation.
* Return storage implemented.


And, yes that is the longest commit message in my life...
Implementation of argument conversion is revised.


git-svn-id: https://root.cern.ch/svn/root/trunk@48537 27541ba8-7e3a-0410-8455-c3a389f83636