[MemProf] Determine stack id references in BitcodeWriter without sorting (#94285)

A cycle profile of a thin link showed a lot of time spent in sort called
from the BitcodeWriter, which was being used to compute the unique
references to stack ids in the summaries emitted for each backend in a
distributed thinlto build. We were also frequently invoking lower_bound
to locate stack id indices in the resulting vector when writing out the
referencing memprof records.

Change this to use a map to uniquify the references, and to hold the
index of the corresponding stack id in the StackIds vector, which is
now populated at the same time.

This reduced the time of a large thin link by about 10%.