Lay the foundations of return value alloc/noalloc.
The JIT's runFunction is unreliable on 32 bit. Thus we provide a similar
functionality in cling to deal with the cases where runFunction cannot.

The underlying issue is that the caller need to provide argument setup
and cleanup, which for 32bit is different than in 64, for example.
The approach we propose should be independent on the concrete platform
implementation, because we pass in the storage and depending on the
return result, we let cling runtime to deside whether and how to alloc
storage.
Keep constness of &; no need for const T* (folds into T*).
We don't need llvm type anymore.
Improve implementation, shorter argument names, less overloads.
Add const to the void* so that the const overloads falls into this case too.
Add var returns test case.
Handle returns, temporary results, independent on JIT's runFunction.

Here we don't want to depend on the JIT runFunction, because of its limitations,
when it comes to return value handling. There it is not clear who provides the
storage and who cleans it up in a  platform independent way.

Depending on the type we need to synthesize a call to cling:
  0) void : do nothing;
  1) enum, integral, float, double, referece, pointer types :
       call to cling::internal::setValueNoAlloc(...);
  2) object type (alloc on the stack) :
       cling::internal::setValueWithAlloc
     2.1) constant arrays:
         call to cling::runtime::internal::copyArray(...)

We need to synthesize later (see RuntimeUniverse.h)
 Wrapper has signature: void w(cling::StoredValueRef SVR)
case 1):
  setValueNoAlloc(gCling, &SVR, lastExprTy, lastExpr())
case 2):
  new (setValueWithAlloc(gCling, &SVR, lastExprTy)) (lastExpr)
  case 2.1):
    copyArray(src, placement, N)
Remove redundant fwd declaration.
Typo.
typo.
Assign if not RS.
We have more accurate results so adjust the test.
The test should be run on 32 bit finally.
Return early in case of empty wrappers for example.
Use a bit better (more descriptive) name.
Update all references of ReturnSynthesizer.