[memprof] Use std::move in ContextEdge::ContextEdge (NFC) (#94687)

Since the constructor of ContextEdge takes ContextIds by value, we
should move it to the corresponding member variable as suggested by
clang-tidy's performance-unnecessary-value-param.

While we are at it, this patch updates a couple of callers.  To avoid
the ambiguity in the evaluation order among the constructor arguments,
I'm calling computeAllocType before calling the constructor.