[libc++] Produce summary reports in compare-benchmarks

This patch adds the ability to produce a summary report with a few KPIs
in the compare-benchmarks script. This is useful to regularly monitor
the progress of the library on these KPIs.

Example usage:

     compare-benchmarks libstdcxx.lnt llvm-20.lnt llvm-21.lnt main.lnt     \
         --series-names "GNU,LLVM 20,LLVM 21,LLVM main"                    \
         --format kpi                                                      \
         --noise-threshold 0.1                                             \
         --meta-candidate 'LLVM'

This would produce a short report showing the evolution of benchmarks
in the given LLVM releases as compared to a GNU baseline.

NOKEYCHECK=True
GitOrigin-RevId: 20c67c75ec3a08938808646f849a98fd6141ce74
1 file changed