[MemProf] Fix reporting with -memprof-matching-cold-threshold (#173327)

With the -memprof-matching-cold-threshold option, we hint as cold
allocations where the fraction of cold bytes is at least the given
threshold. However, we were incorrectly reporting all of the
allocation's contexts and bytes as hinted cold.

Fix this to report the non-cold contexts as ignored. To do this,
refactor out some existing reporting, and also keep track of the
original allocation type for each context in the Trie along with its
ContextTotalSize information. Most of the changes are the change to this
array's type and name.

GitOrigin-RevId: 295a01f36c7ad0f9cacd10e6627993e06d8376e9