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This is the first step to having Clang's runtime libraries fully available for the Carbon toolchain. This PR focuses on the lowest level runtimes, the CRT files and the builtins library. The goal is to intercept Clang runs where it needs these target-dependent pieces to be available, and build them on demand using our Clang-running infrastructure. This avoids most of the subprocess overhead, but there is still some due to missing features in Clang. This requires exporting the sources for these runtimes from the Bazel build, and installing them in our target-independent resource directory. We then build a simplified "build" of these sources within the `ClangRunner` itself to produce the specific artifacts and layout expected by Clang. It also required fixing our use of Clang on macOS to have a default system root in order to successfully compile or link. It also required cleaning up how the `ClangRunner` used target information more generally -- instead of taking the target as a constructor parameter, it manages its target internally and relies on the Clang target-specifying command line flags. I looked at whether we could split this into another layer separate from the `ClangRunner`, but that proved frustratingly difficult to manage. While we support building these on-demand as part of a detected link, that doesn't seem feasible as we don't have the necessary separation between compilation runs of Clang and link runs of Clang. However, I have tried to factor the internals to provide as clear of separation as I could across these. I have also created a stand-alone subcommand to directly build the runtimes which allows for easy testing. It also supports building them into a specific directory, and that directory can in turn be passed to a Clang invocation. This is designed to work both at the API level with `ClangRunner` and at the subcommand level. Currently, the only part of the commandline that is detected and forwarded to the runtimes build is the target. Eventually, the plan is to expand this so that we can build a maximally tailored set of runtimes for a given compilation. The other big TODO here is to actually implement caching storage of these runtimes so they aren't built on every execution. Right now, this uses a somewhat hack-y build of a temporary directory, but this isn't expected to be suitable long-term. Building these runtimes on *every* link makes those commands take approximately 15 seconds with an ASan build like our default development build, and just over 2 seconds in an optimized build. Because of this, I've kept all of this disabled by default for now. The goal is that once caching and some other improvements land, we can enable this by default. --------- Co-authored-by: Jon Ross-Perkins <jperkins@google.com> Co-authored-by: Richard Smith <richard@metafoo.co.uk>