[SampleProfile] Fix non-determinism in promoteMergeNotInlinedContextSamples()

We're seeing non-determinism with loading sample profiles. It seems to
be related to the order in which we merge FunctionSamples in
promoteMergeNotInlinedContextSamples(). Use a MapVector to iterate over
NonInlinedCallSites in the order entries were inserted.

Reviewed By: wenlei, davidxl

Differential Revision: https://reviews.llvm.org/D131592