Restore the capability to manipulate datalayout from the C API

Summary:
This consist in variosu addition to the C API:

  LLVMTargetDataRef LLVMGetModuleDataLayout(LLVMModuleRef M);
  void LLVMSetModuleDataLayout(LLVMModuleRef M, LLVMTargetDataRef DL);
  LLVMTargetDataRef LLVMCreateTargetMachineData(LLVMTargetMachineRef T);

Reviewers: joker.eph, Wallbraker, echristo

Subscribers: axw

Differential Revision: http://reviews.llvm.org/D17255

llvm-svn: 260936