[CSSPGO] Honor preinliner decision for ThinLTO importing

When pre-inliner decision is used for CSSPGO, we should take that into account for ThinLTO importing as well, so post-link sample loader inliner can favor that decision. This is handled by a small tweak in this patch. It also includes a change to transfer preinliner decision when merging context.

Differential Revision: https://reviews.llvm.org/D109088