[memprof] Add access checks to PortableMemInfoBlock::get* (#90121)

commit 4c8ec8f8
  Author: Kazu Hirata <kazu@google.com>
  Date:   Wed Apr 24 16:25:35 2024 -0700

introduced the idea of serializing/deserializing a subset of the
fields in PortableMemInfoBlock.  While it reduces the size of the
indexed MemProf profile file, we now could inadvertently access
unavailable fields and go without noticing.

To protect ourselves from the risk, this patch adds access checks to
PortableMemInfoBlock::get* methods by embedding a bit set representing
available fields into PortableMemInfoBlock.