rustc: Preallocate when building the dep graph

rustc: Preallocate when building the dep graph

This commit alters the `query` function in the dep graph module to preallocate
memory using `with_capacity` instead of relying on automatic growth. Discovered
in #44576 it was found that for the syntex_syntax clean incremental benchmark
the peak memory usage was found when the dep graph was being saved, particularly
the `DepGraphQuery` data structure itself. PRs like #44142 which add more
queries end up just making this much larger!

I didn't see an immediately obvious way to reduce the size of the
`DepGraphQuery` object, but it turns out that `with_capacity` helps quite a bit!
Locally 831 MB was used [before] this commit, and 770 MB is in use at the peak
of the compiler [after] this commit. That's a nice 7.5% improvement! This won't
quite make up for the losses in #44142 but I figured it's a good start.

[before]: https://gist.github.com/alexcrichton/2d2b9c7a65503761925c5a0bcfeb0d1e
[before]: https://gist.github.com/alexcrichton/6da51f2a6184bfb81694cc44f06deb5b