[MemProf] Templatize CallStackRadixTreeBuilder (NFC) (#117014)
Prepare for usage in the bitcode reader/writer where we already have a
LinearFrameId:
- templatize input frame id type in CallStackRadixTreeBuilder
- templatize input frame id type in computeFrameHistogram
- make the map from FrameId to LinearFrameId optional
We plan to use the same radix format in the ThinLTO summary records,
where we already have a LinearFrameId.