We removed access to the DataLayout on the TargetMachine and
deprecated the C API function LLVMGetTargetMachineData() in r243114.
However the way I tried to be backward compatible was broken: I
changed the wrapper of the TargetMachine to be a structure that
includes the DataLayout as well. However the TargetMachine is also
wrapped by the ExecutionEngine, in the more classic way. A client
using the TargetMachine wrapped by the ExecutionEngine and trying
to get the DataLayout would break.
It seems tricky to solve the problem completely in the C API
implementation. This patch tries to address this backward
compatibility in a more lighter way in the C++ API. The C API is
restored in its original state and the removed C++ API is
reintroduced, but privately. The C API is friended to the
TargetMachine and should be the only consumer for this API.