[ThinLTO] Honor -O{0,1,2,4} passed through the libLTO interface for ThinLTO

This was hardcoded to be O3 till now, without any way to change it
without changing the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290682 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed