Recommit "[LTO] Use lto::backend for code generation."

This version of the patch includes a fix for the cfi failures.

(undoes the revert commit 7db390cc)

It also undoes reverts of follow-up patches that also needed reverting
originally:

  * [LTO] Add option enable NewPM with LTOCodeGenerator.
    (undoes revert commit 0a17664b)

  * [LTOCodeGenerator] Use lto::Config for options (NFC)."
    (undoes revert commit b0a8e41c)