trivial: switch AllocationEntry construction to use designated initializer
trivial: switch AllocationEntry construction to use designated initializer

Motivation: extending this struct is a total pain.
PiperOrigin-RevId: 446205183
Change-Id: I41e93c1e5cf22bbbfa3b73c1d5dc886f11175d47

google/tcmalloc,google/tcmalloc,google/tcmalloc