Linking fails with `'ItaniumNodes.def' file not found`

### Description of the bug: While working on reverse introp I've occasionally been running a full compile-link-run test to make sure the code is doing what I think it's doing. This stopped working after [Switch to a Bazel-based runtimes build, and add bootstrapping](https://github.com/carbon-language/carbon-lang/commit/27870892472cbc96324184fb64c2099f95d95f5e). I now get this error: ``` INFO: Invocation ID: c8ff09b8-b4de-4927-9f1f-cac0b719b1cf Computing main repo mapping: Loading: Loading: 0 packages loaded bazel: Entering directory `/usr/local/google/home/nicholasbishop/.cache/bazel/_bazel_nicholasbishop/7a0bf1e4d37f50d2dd7c7e67a3c8032e/execroot/_main/' Analyzing: target //toolchain:toolchain (0 packages loaded, 0 targets configured) Analyzing: target //toolchain:toolchain (0 packages loaded, 0 targets configured) INFO: Analyzed target //toolchain:toolchain (0 packages loaded, 0 targets configured). INFO: Found 1 target... bazel: Leaving directory `/usr/local/google/home/nicholasbishop/.cache/bazel/_bazel_nicholasbishop/7a0bf1e4d37f50d2dd7c7e67a3c8032e/execroot/_main/' Target //toolchain:carbon up-to-date: bazel-bin/toolchain/carbon INFO: Elapsed time: 0.427s, Critical Path: 0.08s INFO: 1 process: 1 internal. INFO: Build completed successfully, 1 total action INFO: Running command line: bazel-bin/toolchain/carbon <args omitted> In file included from /usr/local/google/home/nicholasbishop/.cache/bazel/_bazel_nicholasbishop/7a0bf1e4d37f50d2dd7c7e67a3c8032e/execroot/_main/bazel-out/k8-fastbuild/bin/toolchain/carbon.runfiles/_main/toolchain/install/runtimes/libcxxabi/src/cxa_demangle.cpp:17: /usr/local/google/home/nicholasbishop/.cache/bazel/_bazel_nicholasbishop/7a0bf1e4d37f50d2dd7c7e67a3c8032e/execroot/_main/bazel-out/k8-fastbuild/bin/toolchain/carbon.runfiles/_main/toolchain/install/runtimes/libcxxabi/src/demangle/ItaniumDemangle.h:170:10: fatal error: 'ItaniumNodes.def' file not found 170 | #include "ItaniumNodes.def" | ^~~~~~~~~~~~~~~~~~ 1 error generated. error: failure running `clang` to perform linking: Failed to compile runtime source file 'runtimes/libcxxabi/src/cxa_demangle.cpp' ``` ### What did you do, or what's a simple way to reproduce the bug? `~/tmp/carb/simple.carbon`: ```carbon import Core library "io"; fn Run() -> i32 { Core.Print(1234); return 0; } ``` Run: `./scripts/run_bazelisk.py run //toolchain compile ~/tmp/carb/simple.carbon && ./scripts/run_bazelisk.py run //toolchain -- link --output=/usr/local/google/home/nicholasbishop/tmp/carb/simple ~/tmp/carb/simple.o && ~/tmp/carb/simple` ### What did you expect to happen? _No response_ ### What actually happened? _No response_ ### Any other information, logs, or outputs that you want to share? _No response_