[ThinLTO] Only import for non-prevailing interposable global variables

This logic was added in https://reviews.llvm.org/D95943 specifically to
handle an issue for non-prevailing global variables. It turns out that
it adds a new issue for prevailing glboal variables, since those could
be replaced by an available_externally definition and hence incorrectly
omitted from the output object file. Limit the import to non-prevailing
global variables to fix this, as suggested by @tejohnson.

The bulk of the diff is mechanical changes to thread isPrevailing
through to where it's needed and ensure it's available before the
relevant calls; the actual logic change itself is straightforward.

Fixes https://github.com/llvm/llvm-project/issues/61677

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D146876