LTO: Correct some doxygen comments about API availability

These look like copy/paste errors, and shouldn't have the "prior to"
qualifier.  Each API was introduced at the given values of
`LTO_API_VERSION`.  The "prior to" in other doxygen comments is because
I couldn't easily differentiate between versions 1 and 2 when I added
these comments.

llvm-svn: 235925