[msan] Nit: use init_origins parameter instead of __msan_get_track_origins()...

[msan] Nit: use init_origins parameter instead of __msan_get_track_origins() in InitShadowWithReExec (#86994)

This fixes a nit I had accidentally introduced in
https://github.com/llvm/llvm-project/pull/85142

I don't think the value of __msan_get_track_origins() will change
between the start and end of InitShadowWithReExec, but it's cleaner to
use the parameter.