Due to me not double-checking my PR, an overly eager AI auto-completion
made it into my previous PR :/

Based on review feedback in #160026.

This makes the substitution a lot more clear now that there is no
documentation around %T.

---------

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

The new benchmarking bot is experimental and is very rough on the edges.

This patch replaces the previous `libcxx-compare-benchmarks` wrapper by
a new `compare-benchmarks` script which works with LNT-compatible data.
This allows comparing benchmark results across libc++ microbenchmarks,
SPEC, and anything else that would produce LNT-compatible data.

It also adds a simple script to consolidate LNT benchmark output into a
single file, simplifying the process of doing A/B runs locally. The
simplest way to do this doesn't require creating two build directories
after this patch anymore.

It also adds the ability to produce either a standalone HTML chart or a
plain text output for diffing results locally when prototyping changes.
Example text output of the new tool:

```
Benchmark                              Baseline    Candidate    Difference    % Difference
-----------------------------------  ----------  -----------  ------------  --------------
BM_join_view_deques/0                      8.11         8.16          0.05            0.63
BM_join_view_deques/1                     13.56        13.79          0.23            1.69
BM_join_view_deques/1024                6606.51      7011.34        404.83            6.13
BM_join_view_deques/2                     17.99        19.92          1.93           10.72
BM_join_view_deques/4000               27655.58     29864.72       2209.14            7.99
BM_join_view_deques/4096               26218.07     30520.13       4302.05           16.41
BM_join_view_deques/512                 3231.66      2832.47       -399.19          -12.35
BM_join_view_deques/5500               47144.82     42207.41      -4937.42          -10.47
BM_join_view_deques/64                   247.23       262.66         15.43            6.24
BM_join_view_deques/64000             756221.63    511247.48    -244974.15          -32.39
BM_join_view_deques/65536             537110.91    560241.61      23130.70            4.31
BM_join_view_deques/70000             815739.07    616181.34    -199557.73          -24.46
BM_join_view_out_vectors/0                 0.93         0.93          0.00            0.07
BM_join_view_out_vectors/1                 3.11         3.14          0.03            0.82
BM_join_view_out_vectors/1024           3090.92      3563.29        472.37           15.28
BM_join_view_out_vectors/2                 5.52         5.56          0.04            0.64
BM_join_view_out_vectors/4000           9887.21      9774.40       -112.82           -1.14
BM_join_view_out_vectors/4096          10158.78     10190.44         31.66            0.31
BM_join_view_out_vectors/512            1218.68      1209.59         -9.09           -0.75
BM_join_view_out_vectors/5500          13559.23     13676.06        116.84            0.86
BM_join_view_out_vectors/64              158.95       157.91         -1.04           -0.65
BM_join_view_out_vectors/64000        178514.73    226520.97      48006.24           26.89
BM_join_view_out_vectors/65536        184639.37    207180.35      22540.98           12.21
BM_join_view_out_vectors/70000        235006.69    213886.93     -21119.77           -8.99
```

Without a blank line after the ":", it was rendered on the same
line instead of a new one.

Adds a new lit directive to improve C++ Standard filtering. This is
based on the

[Discourse](https://discourse.llvm.org/t/rfc-improving-c-standard-filtering-in-the-lit-tests/78474)
discussion.