LTO: document LTO_API_VERSION for each API

Adding a doxygen comment for each bit of API to indicate at which
LTO_API_VERSION each was available, manually gleaned from successive
git-blames.  A few notes:

- LTO_API_VERSION was set to 3 at its introduction.
- I've indicated all the API introduced before LTO_API_VERSION was
  around as available "prior to LTO_API_VERSION=3".
- A number of API changes neglected to bump LTO_API_VERSION.  These I've
  indicated as available at the *next* bump of LTO_API_VERSION.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199429 91177308-0d34-0410-b5e6-96231b3b80d8