[memprof] Initialize MemInfoBlock data.

This patch updates the existing default no-arg constructor for
MemInfoBlock to explicitly initialize all members. Also add missing
DataTypeId initialization to the other constructor. These issues were
exposed by msan on patch D121179. With this patch D121179 builds cleanly
on msan.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D122260