Display metadata/codegen legend in --timings graph

[HadrienG2]

The meaning of the blue and purple in the crate build time graph generated by -Ztimings is currently explained out in the documentation, but it would be much better to just make the graph self-explanatory.

[epage]

Unstable support for this is being added in #15923

[Kobzol]

Actually the legend will now also be shown on stable (once the change lands there).

[epage]

Going to go ahead and close this then