[NFC][libLTO] Rearrange declaration in lto.h

Summary:
Rearrange the function declaration in lto.h so they falls in the correct
doxygen group.

Reviewers: tejohnson, bd1976llvm, deadalnix

Reviewed By: tejohnson

Subscribers: mehdi_amini, inglorion, jkorous, dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67565

llvm-svn: 371900