Start building Clang runtimes on-demand (#5338)

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>

toolchain/driver/clang_subcommand.cpp

@@ -12,6 +12,41 @@
 namespace Carbon {
 
 auto ClangOptions::Build(CommandLine::CommandBuilder& b) -> void {
+  b.AddStringOption(
+      {
+          .name = "prebuilt-runtimes",
+          .value_name = "PATH",
+          .help = R"""(
+Path to prebuilt target runtimes for Clang.
+
+If this option is provided, runtimes will not be built on demand and this path
+will be used instead.
+)""",
+      },
+      [&](auto& arg_b) { arg_b.Set(&prebuilt_runtimes_path); });
+  b.AddFlag(
+      {
+          .name = "build-runtimes",
+          .help = R"""(
+Enables on-demand building of target-specific runtimes.
+
+When enabled, any link actions using `clang` will build the necessary runtimes
+on-demand. This build will use any customization it can from the link command
+line flags to build the runtimes for the correct target and with any desired
+features enabled.
+
+Note: this only has an effect when `--prebuilt-runtimes` are not provided. If
+there are no prebuilt runtimes and building runtimes is disabled, then it is
+assumed the installed toolchain has had the necessary target runtimes added to
+the installation tree in the default searched locations.
+)""",
+      },
+      [&](auto& arg_b) {
+        // TODO: Once runtimes are cached properly, the plan is to enable this
+        // by default.
+        arg_b.Default(false);
+        arg_b.Set(&build_runtimes_on_demand);
+      });
   b.AddStringPositionalArg(
       {
           .name = "ARG",
@@ -46,9 +81,9 @@ ClangSubcommand::ClangSubcommand() : DriverSubcommand(SubcommandInfo) {}
 // add more.
 // https://github.com/llvm/llvm-project/blob/main/clang/tools/driver/driver.cpp
 auto ClangSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
-  std::string target = llvm::sys::getDefaultTargetTriple();
-  ClangRunner runner(driver_env.installation, target, driver_env.fs,
-                     driver_env.vlog_stream);
+  ClangRunner runner(
+      driver_env.installation, driver_env.fs, driver_env.vlog_stream,
+      /*build_runtimes_on_demand=*/options_.build_runtimes_on_demand);
 
   // Don't run Clang when fuzzing, it is known to not be reliable under fuzzing
   // due to many unfixed issues.
@@ -61,7 +96,27 @@ auto ClangSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
     runner.EnableLeakingMemory();
   }
 
-  return {.success = runner.Run(options_.args)};
+  std::optional<std::filesystem::path> prebuilt_resource_dir_path;
+  if (!options_.prebuilt_runtimes_path.empty()) {
+    prebuilt_resource_dir_path = options_.prebuilt_runtimes_path.str();
+    // TODO: Replace the hard coded `clang_resource_dir` subdirectory here with
+    // an abstraction that manages the layout of the built runtimes.
+    *prebuilt_resource_dir_path /= "clang_resource_dir";
+  }
+
+  ErrorOr<bool> run_result =
+      runner.Run(options_.args, prebuilt_resource_dir_path);
+  if (!run_result.ok()) {
+    // This is not a Clang failure, but a failure to even run Clang, so we need
+    // to diagnose it here.
+    CARBON_DIAGNOSTIC(FailureRunningClang, Error,
+                      "failure running `clang` subcommand: {0}", std::string);
+    driver_env.emitter.Emit(FailureRunningClang, run_result.error().message());
+    return {.success = false};
+  }
+
+  // Successfully ran Clang, but return whether Clang itself succeeded.
+  return {.success = *run_result};
 }
 
 }  // namespace Carbon