Make the C++ LTO API easier to use from C++ clients.

Make the C++ LTO API easier to use from C++ clients.

Start using C++ types such as StringRef and MemoryBuffer in the C++ LTO
API. In doing so, clarify the ownership of the native object file: the caller
now owns it, not the LTOCodeGenerator. The C libLTO library has been modified
to use a derived class of LTOCodeGenerator that owns the object file.

Differential Revision: http://reviews.llvm.org/D10114

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238776 91177308-0d34-0410-b5e6-96231b3b80d8