Sink undesirable LTO functions into the old C API

We want to encourage users of the C++ LTO API to reuse memory buffers instead
of repeatedly opening and reading the same file contents.

This reverts commit r212305 and implements a tidier scheme.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212308 91177308-0d34-0410-b5e6-96231b3b80d8