CallStackRadixTreeBuilder::build takes the parameter
MemProfFrameIndexes by value, involving copies:
std::optional<const llvm::DenseMap<FrameIdTy, LinearFrameId>>
MemProfFrameIndexes
Then "build" makes another copy of MemProfFrameIndexe and passes it to
encodeCallStack for every call stack, which is painfully slow.
This patch changes the type to a pointer so that we don't have to make
a copy every time we pass the argument.
Without this patch, it takes 553 seconds to run "llvm-profdata merge"
on a large MemProf raw profile. This patch shortenes that down to 67
seconds.
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