[MemProf] Update hot/cold information after importing

The support added by D149215 to remove memprof metadata and attributes
if we don't link with an allocator supporting hot/cold operator new
interfaces did not update imported code. Move the update handling later
in the ThinLTO backend to just after importing, and update the test to
check this case.

Differential Revision: https://reviews.llvm.org/D150295