Reland "[ThinLTO] Populate declaration import status except for distributed ThinLTO under a default-off new option"

[minglotus-6]

TODO: fix build-bot failure (maybe set thinlto-threads to 1)

Original Commit Message:

The goal is to populate declaration import status if a new flag -import-declaration is on.

• For in-process ThinLTO, the declaration status is visible to backend function-import pass, so FunctionImporter::importFunctions should read the import status and be no-op for declaration summaries. Basically, the postlink pipeline is updated to keep its current behavior (import definitions), but not updated to handle declaration summaries. Two use cases (better call-graph sort or cross-module auto-init) would use this bit differently.

• For distributed ThinLTO, the declaration status is not serialized to bitcode. As discussed, [ThinLTO][Bitcode] Generate import type in bitcode #87600 will do this.

[minglotus-6]

Will create PR from branch 'users/minglotus-6/spr/summary2' to keep commit history