Toolchain configuration can't produce stripped binaries

[mmdriley]

On macOS, at 9ed1c5cee9c3326178b80a35a8f217257971b410, with or without .bazeliskrc wired to 4.0.0:

$ bazelisk build //executable_semantics:executable_semantics.stripped
[...]
ERROR: /Users/mattdr/src/carbon-lang/executable_semantics/BUILD:11:10: Stripping executable_semantics/executable_semantics.stripped for //executable_semantics:executable_semantics failed: (Exit 1): llvm-strip failed: error executing command /private/var/tmp/_bazel_mattdr/d9970d429502ef17c1da219b3a765faf/external/bootstrap_clang_toolchain/bin/llvm-strip

Use --sandbox_debug to see verbose messages from the sandbox llvm-strip failed: error executing command /private/var/tmp/_bazel_mattdr/d9970d429502ef17c1da219b3a765faf/external/bootstrap_clang_toolchain/bin/llvm-strip

Use --sandbox_debug to see verbose messages from the sandbox
OVERVIEW: llvm-strip tool

USAGE: llvm-strip [options] inputs...
[...]

We can confirm with aquery that Bazel is invoking llvm-strip with no arguments:

$ bazelisk aquery 'mnemonic("CcStrip", //executable_semantics:executable_semantics)'
INFO: Analyzed target //executable_semantics:executable_semantics (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
action 'Stripping executable_semantics/executable_semantics.stripped for //executable_semantics:executable_semantics'
  Mnemonic: CcStrip
  Target: //executable_semantics:executable_semantics
  Configuration: darwin-fastbuild
  ActionKey: f5e061b91160a2d1693c98ea4144f0090598e0d626416aa402d52095236980ed
  Inputs: [bazel-out/darwin-fastbuild/bin/executable_semantics/executable_semantics, bazel-out/darwin-opt-exec-2B5CBBC6/internal/_middlemen/external_Sbazel_Ucc_Utoolchain_Cempty]
  Outputs: [bazel-out/darwin-fastbuild/bin/executable_semantics/executable_semantics.stripped]
  Environment: [LANG=en_US.UTF-8]
  Command Line: (exec /private/var/tmp/_bazel_mattdr/d9970d429502ef17c1da219b3a765faf/external/bootstrap_clang_toolchain/bin/llvm-strip)

By comparison, if I create another Bazel workspace elsewhere and let it pick up the local toolchain, we see:

$ bazelisk aquery 'mnemonic("CcStrip", //:hello)'
INFO: Analyzed target //:hello (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
action 'Stripping hello.stripped for //:hello'
  Mnemonic: CcStrip
  Target: //:hello
  Configuration: darwin-fastbuild
  ActionKey: 010f253cf13d63c4f2509c10f0ee0b534d93c03d0c65d24a4398b1c6e04d6068
  Inputs: [bazel-out/darwin-fastbuild/bin/hello, bazel-out/darwin-opt-exec-2B5CBBC6/internal/_middlemen/external_Slocal_Uconfig_Ucc_Cempty]
  Outputs: [bazel-out/darwin-fastbuild/bin/hello.stripped]
  Command Line: (exec /usr/bin/strip \
    -S \
    -o \
    bazel-out/darwin-fastbuild/bin/hello.stripped \
    bazel-out/darwin-fastbuild/bin/hello)

that configuration appears to come from: https://github.com/bazelbuild/bazel/blob/073ae810e4f61f56f68a31d5ee95f18633d176cc/tools/osx/crosstool/cc_toolchain_config.bzl#L296-L311

meanwhile, we configure the strip action but don't attach any flag_sets, so I guess Bazel just thinks it can run it with no flags. https://github.com/carbon-language/carbon-lang/blob/2111b859da527d02c7eda7bcc1558b25a57f20f3/bazel/cc_toolchains/clang_cc_toolchain_config.bzl#L97-L100