This depends on https://github.com/compiler-explorer/infra/pull/1936
which builds the runtimes when installing the Carbon toolchain. Then
this PR passes `--prebuilt-runtimes` to the `carbon link` step in order
to use them. The result is that compiler explorer no longer times out
when trying to link a binary with the Carbon toolchain.
The carbon.ts file is refactored a little in an attempt to improve
clarity about what is going on, and some comments are added throughout.
Fixes https://github.com/carbon-language/carbon-lang/issues/6603.
edf9bd889e