[libc++] Add a job that runs the benchmarks as part of the CI

[ldionne]

[libc++] Add a job that runs the benchmarks as part of the CI

This will make it easier to see the performance impact of individual
patches. This is a first step towards better performance tracking, but
many things still need to be figured out:

• We should get a dedicated set of machines to run these tests on
• We could probably reuse regularly-generated baseline results published
  to LNT instead of re-generating the baselines in this job every time.

[ldionne]

test edit commit via API

[ldionne]

/libcxx-bot benchmark libcxx/test/benchmarks/join_view.bench.cpp libcxx/test/benchmarks/hash.bench.cpp

Benchmark results:

Benchmark                              Baseline    Candidate    Difference    % Difference
-----------------------------------  ----------  -----------  ------------  --------------
BM_Hash/uint32_random_std_hash/1024      363.18       362.45         -0.74           -0.20
BM_Hash/uint32_top_std_hash/1024         363.50       364.26          0.76            0.21
BM_join_view_deques/0                     17.34        17.29         -0.05           -0.29
BM_join_view_deques/1                     23.45        23.43         -0.02           -0.10
BM_join_view_deques/1024                7354.99      7622.90        267.91            3.64
BM_join_view_deques/2                     28.33        29.24          0.92            3.23
BM_join_view_deques/4000               38507.79     39075.92        568.12            1.48
BM_join_view_deques/4096               40568.47     39302.25      -1266.22           -3.12
BM_join_view_deques/512                 3372.40      3252.08       -120.32           -3.57
BM_join_view_deques/5500               52833.82     55038.01       2204.19            4.17
BM_join_view_deques/64                   371.55       364.72         -6.84           -1.84
BM_join_view_deques/64000            1132596.67   1114455.72     -18140.95           -1.60
BM_join_view_deques/65536            1213556.56   1257778.02      44221.46            3.64
BM_join_view_deques/70000            1354353.37   1435964.08      81610.71            6.03
BM_join_view_in_vectors/0                  7.17         7.19          0.02            0.27
BM_join_view_in_vectors/1                 11.84        11.81         -0.03           -0.22
BM_join_view_in_vectors/1024            3698.59      3638.25        -60.33           -1.63
BM_join_view_in_vectors/2                 10.31        10.32          0.01            0.08
BM_join_view_in_vectors/4000           15672.18     18722.86       3050.68           19.47
BM_join_view_in_vectors/4096           15971.69     18405.45       2433.76           15.24
BM_join_view_in_vectors/512             1811.43      1807.01         -4.42           -0.24
BM_join_view_in_vectors/5500           22703.60     21713.82       -989.78           -4.36
BM_join_view_in_vectors/64               228.15       227.82         -0.33           -0.15
BM_join_view_in_vectors/64000         333670.88    286360.85     -47310.03          -14.18
BM_join_view_in_vectors/65536         309820.17    289218.06     -20602.11           -6.65
BM_join_view_in_vectors/70000         339185.03    329616.72      -9568.31           -2.82
BM_join_view_out_vectors/0                 0.93         0.93          0.00            0.02
BM_join_view_out_vectors/1                 4.99         4.99         -0.00           -0.05
BM_join_view_out_vectors/1024           4032.52      4115.79         83.28            2.07
BM_join_view_out_vectors/2                 8.20         8.12         -0.09           -1.05
BM_join_view_out_vectors/4000          18296.18     16280.00      -2016.19          -11.02
BM_join_view_out_vectors/4096          18476.16     16397.04      -2079.12          -11.25
BM_join_view_out_vectors/512            2022.21      1973.60        -48.61           -2.40
BM_join_view_out_vectors/5500          22438.25     22383.05        -55.21           -0.25
BM_join_view_out_vectors/64              245.87       245.85         -0.02           -0.01
BM_join_view_out_vectors/64000        280803.15    278943.67      -1859.48           -0.66
BM_join_view_out_vectors/65536        345450.46    285769.10     -59681.36          -17.28
BM_join_view_out_vectors/70000        314685.18    313704.59       -980.59           -0.31

[ldionne]

Closing this since after a whole lot of trial and error, I managed to make it work with a comment-triggered job instead.