Modernize the C++ APIs for creating LTO modules.

This is a continuation of r253367.

These functions return is owned by the caller, so they return
std::unique_ptr now.

The call can fail, so the return is wrapped in ErrorOr.

They have a context where to report diagnostics, so they don't need to
take a string out parameter.

With this there are no call to getGlobalContext in lib/LTO.

llvm-svn: 254721